Simpler for whom? The user who has to manage N passwords for N services, instead of one password? I'd rather Twitter's staff suffer a moderate amount of pain once than have every user suffer lesser pains continuously.

Programmers (myself included) too often fail to consider whether "technically simpler" is "actually simpler". It is one of the things that ensures the poor quality of software. We'd do well to understand that our job is, first and foremost, to make life easier for the people using our software. This may require martyrdom from time to time.

Besides, OAuth isn't all that complicated. Distilled, it's similar to your idea but with several additional benefits, not the least of which is that there are existing implementations to help you on your path.