AppImages are only as self-contained as the author put effort into making them self-contained. There's also upper limits to how self-contained they are. While some terminal and bitmap only X11 app can be compiled as static binaries, anything that depends on system libraries needs to be compiled with an older version of glibc. The best example is libGl (GLX or EGL) for hardware 3D acceleration or libvdpau for hardware media decoding. You can't just bundle those, you have to use the system ones. Using any system library forces you to use glibc (AppImage don't work on Alpine). OpenSSL and a few other a libs you usually want to use the system one and have a built-in fallback because of security concerns.

Making perfect AppImages is often possible, but the automated tooling isn't smart enough. A proper AppImage (this one is by me) look like this: https://github.com/Elv13/reclaimail/blob/master/docker-edito... . Obviously this doesn't scale very well to projects with 300 dependencies like Digikam. My NeoVIM appimage linked above "really, really" bundles all dependency and compile your NeoVIM config to luajit bytecode. It's 3.9mb compared to the upstream one which is 15mb without any config. Note than 0.7mb of that 3.9 is the spellcheck dictionary, 0.4 my enormous config, 0.5 the AppImage overhead and 0.7 all the legacy plugins still written in vimscript.