Caching produces confusion, is my guess. You have to know it's there to realize how to fix the "oops, I made changes and they aren't showing up" issues.

Invalidation in the case of a blog is simple enough that the caching should be completely transparent.

Should be, but Wordpress is built to accomodate everything, and a caching layer baked in by default would screw up so many plugins, half the community wouldn't know what to do with their blogs.

If it was baked in from the beginning, no problem, but there is so much plugin momentum in Wordpress now that throwing a caching layer on top of it would confuse the hell out of a lot of people who just want to write words.