Only because of the enormous efforts put in by debian package maintainers and it's infrastructure.

If you're a an indie developer wanting your application to run on various debian based distros but the debian maintainers won't package your application, that's when you'd see why it's called DLL hell, how horribly fragmented the Linux packaging is and why even steam ships their whole run time.

Everything inside Debian is fine. That's most of the ecosystem apart from the very new stuff that isn't mature enough yet. Usually the reason something notable stays out if Debian long term is when that thing has such bad dependency hygiene that it cannot easily be brought up to standard.