Also, I can focus only on the solutions and features I need. I often don't have to build for the super general case, which often results in a simpler framework that helps rather than hinders.
Frameworks are not to be avoided, but generic frameworks are very dangerous, so usually a custom framework, built with generic libraries, is the better choice.
I read somewhere (wish I could remember where, bizarrely I think it was on paper) that the very definition of a framework over a library is that inversion of control - ie your application runs inside the framework - you are beholden to its lifecycles and view of the world.
When on the JVM, I use scala - and I have found very little utility for frameworks. Libraries - oh yes, very much so.
Main points: two frameworks can be very hard to use together, unless designed to be (both keep trying to keep the control), frameworks are a lot harder to use than library, as they impose their whole world view, and frameworks encourage tight coupling (there are about 3 patterns from Ralph's book with Erich Gama, Richard Helm and John Vlissides that have this kind of problem to solve).