You're right: the temptation to build from scratch is strong, and probably inappropriate. I think we're dealing with two fundamental problems:

1. Most developers don't grok JavaScript. The object model (prototype-based) really throws off a lot of folks, and the fact that JavaScript can't seem to make up its mind if it's classical or prototypical doesn't help. There are numerous other problems as well made evident, and worse, by the lousy code you see floating around.

2. Fixing and improving JavaScript moves at a glacial pace due to the rate of turnover in browser versions. Chrome has it right insofar that they make upgrades automatic, easy and transparent. As always IE is a major problem, and MS' refusal to adopt a Chrome-style upgrade process isn't helping any.