Otto describes why neither page caching, nor persistent object caching are part of the WordPress core, and why they probably never will be. The main reasons are the way WordPress is architected, and the way it is used by the majority.
If you’re a person writing a blog that gets less than 1000 hits a day, caching ain’t going to do much for you.
And my favorite, about why object cache doesn’t (and probably shouldn’t) use the database for persistent storage:
Trust the MySQL query optimizer, it’s smarter than you are.
