People have tried to provide a packaging format that would allow apps to declare their depencies in a distribution neutral manner[1]. It was, uh, not a huge success.

Let's take an extreme example. If I build an app on Ubuntu and then try to run it against the system libraries on Alpine, it'll fail, because Alpine is built against a different libc. We can simply declare Alpine out of scope and only support glibc based distributions, but if we want good cross-distribution support we're still going to be limited to what's shipping in the oldest supported version of RHEL. So let's skip that problem by declaring LTS distros out of scope and only target things shipped in the last 3 years - and now apps can't target any functionality newer than 3 years old, or alternatively have to declare a complicated support matrix of distributions that they'll work with, which kind of misses the point of portability.

In an ideal world distributions would provide a consistent runtime that had all the functionality apps needed, but we've spent the past 20 years failing to do that and there's no reason to believe we're suddenly going to get better at it now. The Flatpak approach of shipping runtimes isn't aesthetically pleasing, but it solves the problem in a way that's realistically achievable rather than one that's technically plausible but socially utterly impossible.

Flatpak is a pragmatic solution for an imperfect world - just like most good engineering is.

Edit to add: one of the other complexities is that dependencies aren't as easy to express as you'd like. You can't just declare a dependency on a library SONAME - the binary may rely on symbols that were introduced in later minor versions. But you then have to take into account that a distribution may have backported something that added that symbol to an older version, and the logical conclusion is that you have to expose every symbol you require in the dependencies and then have the distribution resolve those into appropriate binary package dependencies, and that metadata simply doesn't exist in every distribution.

[1] http://refspecs.linux-foundation.org/LSB_4.1.0/LSB-Core-gene...