I'm currently busy with completing their questline. I've already finished all other guilds spread trough a few characters.
Why the hell do you become a member of the circle so early? I barely did anything. I killed a tiger and collected a part of an old axe together with someone. Quite dissapointing.

What happened to the awesome oblivion or morrowind guild system?

I believe the "Rationalization" answer is that you found out the Companions Circle is composed of werewolves when the Silver Hand attacked. So, they want you to join in to keep their secret.

A less cynical answer is that you also proved your loyalty by helping Farkas kill a bunch of werewolf hunters.

I "pad" out the adventures, of couse, by doing Radiant Quests and dungeon exploring so I'm already a reknowned warrior by the time I'm invited.

You finished all the other guilds, run across a simplified, entry level guild, and want to know what happened to the guild system?

I smell something....

Soooo, even the guards notice your a badass adventurer, you've been "knighted" by multiple kings, you've killed multiple dragons with witnesses, you've got better armor than Jorgen Greymane.... and you wonder why the fighters guild might want you as a "premium level" member?

At a certain level, the question kinda becomes "why are you slumming it with those losers" not "why do they make you their leader so quickly."

The fact that the OP finished other guilds has nothing to do with his question.

Aside from the reasons Charlemagne19 gave, there is probably another, more depressing reason. There are only six quests in the official Companions quest line.

No offense, but I played Oblivion. Even if there's only six storyline quests, the Radiant Quests (while optional) are about as big as the quests from Oblivion.

Seriously, in Oblivion you had quests like, "Steal the book of the Mage Guild head"

UPSTAIRS

and "find the missing mage member" which was DOWNSTAIRS.

They weren't exactly overwhelmingly designed

I don't remember the OB quests too well (been 5 years since I played it) but I do recall the guild stuff in OB didn't seem to take too long to do, Mages especially. At least the College quests in Skyrim had a good story to them.

I do think things might have been better if all of the Guilds had quests like the Thieves Guild.

Maybe the Companions might have expanded their influence and opened up Chapter Houses.

The Mage college starting to open Potion shops or something.

Who knows.

You're the superhero badass Dragonborn! Who wouldnt want you?
This is just a personal guess, but it feels as though maybe people were meant to do the first few companions quests at quite a low level, so maybe they were impressed by what a good warrior you are? I did their first quest to go fetch their thingie at an early level and it was kinda hard. Perhaps it is just that you impressed them.
Also your rep as a DragonSlayer will do nothing but enhance their popularity.

Besides, it's not like the Companions are a terribly large group. Most people don't get to join but most people can't slaughter 14 or 15 Silver Hand either.

Ria is like, "I killed a BEAR yesterday. A BEAR."

Edit:

Admittedly, there were times I feared bears more than dragons.

bears are probably in the top three most dangerous things in skyrim, akatosh should have brought the ursakiin into the world to steal all the bears souls so they wouldn't respawn to reek more havok

That would be terrible.

Well it's not always about the toys. Sometimes it's about doing good! I didn't much like becoming a werewolf, but I was able to cure myself and some of the others, so you can turn them into a "whiter" guild.

I happened to join the companions early on once in Whiterun, but got derailed by Sam Guevenne and suddenly embarked on a whole Markarth storyline. I finally helped the companions, but then.. after another quest or two, they wanted to join me in the circle. Guess what? I refused at first and mulled on it a bit. There are many quests in this game I don't have to linearly follow just one to completion right away.

So basically, it wasn't a very short questline for me, since the game in general isn't that short.

edit: I think by the time I did do that quest to get the first fragment of Wuuthrad with Farkas, he kept falling a bunch of times. I was already at a level where it was Draugr Wights or Scourges, I think. He wasn't equipped well for it. I can understand why I was invited to the Circle after that.

Ya the companions quest line was very short....I want the fighters guild back.

Heh. I sometimes think thee n me are the only two who just don't push through questlines as if the sky will fall if we do a bunch of other stuff in the middles....

If I recall correctly, there was even a moment when me and Farkas got overwhelmed by Draugrs and bandits and he said "I'm getting out of here!" It's obvious why my character rose up so quickly. Not even Farkas can hang.

The mage guild initiation quests was supposed to be annoying to show the bad state the guild was in.
Think Skyrim is more like Morrowind who had even more pointless quests, benefit in Skyrim is that the radiant quests is random like in Daggerfall, remember picking the mushrooms for the Mage guild starting quest in the starting village, and not remember doing it as it was an automatic thing to do.

The randomness of the Radiants can factor in too.

I've had my first two companion quests be a brawl with Ulbearth, and fighting a lone fugitive bandit outside Whiterun.
I've had the same quests spin out to retrieve an axe from a Falmer nest, then go rescue someone from a Forsworn base.

The Falmer quest and all retrieving of family heirlooms is always from Skjor, I think. Weird, since he considers his own family a bunch of milk drinkers.

Farkas gives the brawl quests (also Forsworn and Vampire quests.. basically, "trouble in whiterun"). Aela deals with animal invasion problems. Vilkas has only given me kidnapping rescues. Speaking of which, that one offered a funny moment, where one of the battle-born women got kidnapped. Once I entered the bandit camp though and took out some people, she joined at the last second and slit the bandit chief's throat from behind. I no longer disliked the "great clan battle born" from that point.

All the faction quest lines involve a relatively small number of named quests - but in some cases the quest itself can be pretty long. Reading the official guide, I'm wondering if the original idea was that each new main faction quest needed a new radiant quest to be completed, so that you'd have to do several radiant quests. Then someone decided that people might be put off and dropped the requirement to a single radiant quest for that faction. Or the radiant system is bugged. But one radiant quest per main faction quest would make more sense: you'd have to prove your overall worth.