Hooray for WordPress!
That WP should have built-in caching, I cannot argue, but once the choice is made to use it, it becomes the chooser's problem.
Why? Do people with least technical knowledge just happen to build Wordpress-based sites attracting billions of eyeballs?
The target market of "people technical enough to download, roll out and host their own version of Wordpress, but not technical enough to find and roll out a plugin" is fairly small.
They could probably package WP-Cache with a default distribution of WordPress, but since that requires allowing write permissions on some directories, I suspect they're erring on the side of security.