Because programmers find a good way to do something then apply it to everything. It becomes the one true way, a dogma, a rule. Like how OOP was the best thing ever for everything, and just now 30 years later is proven to be actually bad. At least appimage is more like DOS-s “just unzip and run it” then “download another 500MB of useless stuff because the program depends on 1 20kB file in it”.
That said, well made libraries are good. As in those that have a stable API so versions don’t matter that much.