I am assuming that you're judging the behavior by the minimal example in the top 1% of the site. If you scroll further, you will notice how extensible it is to support not only month and date navigation but many other customizations.
I also assumed it was limited in such ways. Mostly because it is extraordinarily rare that anyone bothers to enable those. I'd much, much, much rather have a singular, consistent full UX available to me instead of defaulting to some "simple elegant" artificially stunted version.