> What they typically don't do is work with [...], package managers

Well they do since they provide a Deb package. One can wonder why this one and not another, and the answer is probably Ubuntu being one of the most widespread distro out there and especially for beginners.

> Mostly it does not involve the words flatpak or snap. Just not a thing.

I think you're underestimating this a bit. Not saying it's a standard or a default, far from it, but commercial applications are sometimes released as flatpaks first and maybe a .deb package second (Geforce Now as a flatpak only, Slack as a snap first, deb second)