Why you can't jump while sprinting ?
Why you can't use a spell while in mid air ?
Why you can't jump while you are using a spell ?
Why you can't shout while falling ?
Why you don't get on fire when you jump inside fire ?


I really wonder why these things are.

Game design, pretty much sums up everyone of your questions.

.translateTo() negates all other movement controls until its done?
Because you're in motion.
You can't be in motion.
Because you're in motion.
It's not a real flame. Though you can make a camp fire if you want(PC only) and put a real flame there.

Give me some details don't be dry. Explain to me. Elaborate.
What are the reasons for these happening/being ?

Only Todd Howard could tell but he doesn't post so often here.

What more honestly is there to know? It was made this way and so we cannot shoot spells when we are in the air and so on. There is no real reason none that can give any answer to you question because nothing you asked is effected by lore or the game world.

Here is some advice the devs do look here on these forums on occasion if you wish to do these things request they are put in.

I can't give a better explanation(you didn't read it anyway). All I know is each of those actions involve the character not moving(falling = moving). The last I know is fact.

Now you seem to get my meaning!
These aren't explainable by neither the lore or game world,and actually no one knows why the game puts these limitations on the player. There aren't even gameplay reasons for them to be like that.
They are just unexplainable limitations that doesn't make sense.

Yep I did get that when the player is running or falling is moving,and I see the connection of movement with these limitations. The matter is WHY they made it so you can't do these actions,while also doing the other ones ? What's the reasoning behind it ?
It doesn't make sense for example not being able to shout,to use your voice while falling. Your mouth is still free to do what you want.