In a recent post to his blog Brandon Savage revisits one of his older posts taking about why he thinks every developer should write their own framework. In this more recent post that brings up another idea - people who write frameworks together are very smart.
I’ve always known that a group of very committed people could be much more effective at producing a good product than I could be on my own, and for a simple reason: the collective mind usually irons out ideas that work much more quickly than a solitary developer or thinker. [...] The point of this article is to encourage developers to experiment, but remind them that at the same time, there are already existing solutions that work exceptionally well for many if not most needs.
He still recommends developing a framework as a learning tool, but maybe not to use in production applications. There's bigger, more well-developed frameworks out there that can do the job and have the benefit of a "hive mind" of talented developers with perspectives you might not even think of.