Since you can't lobotomize someone when they quit, it's impossible to prevent someone from using proprietary knowledge and insights at their new job, even with a nondisclosure agreement in place. IP transfer doesn't always happen in a way that's overt and easily prosecutable, like stealing confidential blueprints or explicitly telling your new employer trade secrets.

Well, I think that in most cases, a fair amount of that knowledge qualifies as general knowledge about the problem space, notwithstanding that employers would like to paint it all as proprietary.

Still, there is a little truth in what you say. But we in California think that that is a price very much worth paying in exchange for the more vibrant economy afforded by outlawing noncompetes.