I'd content that it's out of ignorance, and I don't mean that in a mean or nasty way. I've heard lots of pushback from PHP devs that it's way easier to update than sites written in other languages are, but I think it's genuinely due to a lack of understanding of
why those languages recommend other upgrade processes. Those processes solve real, genuine problems
that also affect PHP, but they're dismissed as overkill or enterprisey or overly complicated.
And all that may be true for a trivial website. If you've written a personal project with 10,000 hits per year, YOLO. Go for it. The odds of it affecting one of those users is vanishingly tiny, and so what if it does? But if you're hosting something like a Wordpress site for a large company with lots of traffic, it's crucial to understand why "just rsync the files over" is not an acceptable deployment method.