The problem is there is no guarantee they will play nicely together, not clash, or provide similar and compatible APIs. JavaScript developers are running into a very similar set of problems and having everyone cobble together their own solutions is pretty clearly not an answer, which obvious to those who have tried. You may already know what is being abstracted by some of the more expansive frameworks (Cappuccino, SproutCore, Backbone, etc) but there are a lot of people who do not know or do not want to deal with the differences in browsers, etc.

I'm assuming that you don't consider jQuery or its ilk restrictive as far as abstracting the DOM. I don't know of many application developers who would prefer to deal with the low-level DOM API (which differs across browsers, etc) instead of a library like jQuery. There are many people who approach application structure from a similar viewpoint. Instead of rolling their own and running into problems scaling and maintaining their applications, they can use a framework that provides an abstraction that is proven to work. Everyone tries to roll their own pet framework project because that's where the glory is. That's not necessarily the best thing for the future of the web.