WordPress' inability to stand up to even moderate levels of traffic is well known enough at this point that the product really should come with some kind of caching solution built in, IMO. Yes, there are caching plugins, but the users who need them the most are also the ones least likely to know they need them.
Software users not having a comprehensive understanding of the technical complexities of their software is nothing new. It is, in fact, the status quo , as anybody who does IT work for a living can attest.
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.
On one end of the spectrum, you have non-technical people going for wordpress.com accounts that are hosted by Automattic. On the other hand you have fairly technical high-traffic users like New York Times or CNet hiring either their own technical staff (or, once again, outsourcing that to Automattic).
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.