Because it's more fun, primarily. The reason I choose any methodology in the game over another. I'm not playing Dark Souls. I'm dropping into the mind of an imagined person who I have concieved of in a fair amount of detail, but not every detail.
Roll and assigning isn't "sloppy", it's semi-randomized. It takes your basic layout, and gives you some small surprises to integrate into that layout.
As to your statement about class norms, no, that has never been my experience at all. Literally not once in 25 years or so of playing DND and it's derivatives have I seen a table full of class stereotypes when rolling, and rarely have I seen it happen even with point buy.