This is probably because many major developers say they at best would gain ~1% more performance out of the change for a decently large investment of man hours. The opinion of those devs (whether you or I agree with them or not) is that time can be better spent making other improvements to their codebases.

"Pretty much none"? It's more likely that "pretty much" every application that would actually benefit from being 64 bit is already ported.

Yes, like chromium, something I've been using as a 64bit native app since 2009 on Linux.