Because it's easier at the beginning. Doing a big application from scratch requires doing stuff like requirements analysis, which means you need to understand the universe of tools available to make good decisions. Then you need to prototype for a while before settling on the final design (i.e. your custom framework).
With rails (or whatever) you get something that works immediately, which feels safer to a lot of folks who don't have the deep toolkit expertise required above. The problem is that while the framework works, you don't actually understand it any better than you do the toolkits, it's just handling stuff for you with (what you understand as) voodoo. So when it comes time to make big design changes in the future, the toolkit folks know exactly where to start, while the framework people are completely lost.
I'm not completely against frameworks, but I've seen so many misused that I'm very wary. A stupidly-applied library is usually a point-source bug that can be fixed in one place. A stupidly-applied framework infects the whole application with its mess.
With rails (or whatever) you get something that works immediately, which feels safer to a lot of folks who don't have the deep toolkit expertise required above. The problem is that while the framework works, you don't actually understand it any better than you do the toolkits, it's just handling stuff for you with (what you understand as) voodoo. So when it comes time to make big design changes in the future, the toolkit folks know exactly where to start, while the framework people are completely lost.
I'm not completely against frameworks, but I've seen so many misused that I'm very wary. A stupidly-applied library is usually a point-source bug that can be fixed in one place. A stupidly-applied framework infects the whole application with its mess.