And out comes the Thieves Guild questline, with the best explanation in the game
In short, Brynjolf said, "I'm probably the best thief, but I'm not by all accounts the best leader"
it still doesn't explain the ridiculously short time until he said that (around 4-5 quests tops), but it makes a hefty amount of sense: the most veteran guy may not probably be the most leader-like
Also, we don't get to be the Guild Master by just finishing the "main" quest line. Oh no, you need to work your fingers to the bone with petty crimes first. At least it makes us work for the title, and by the time we DO get the title, we know that we've worked enough to be a respectable guy around the house indeed.
I like it. Thieves Guild once again does not fail me

