I mean there's a lot of things that make life difficult with Android.
* If you want most hardware to keep on working, no kernel updates are possible after the manufacturer ends support.
* Root support and service managers need to be installed aftermarket using the unofficial solution Magisk.
* No vendor kernel out of the box supports kernel compile time flags to enable the infrastructure to run Linux containers using Docker, Podman or LXC.
* Self-invented file hierarchy with a lot of restrictions that make sense from an Android UI perspective but are prohibitive when used as a regular distro.
* Since Android and drivers use bionic libc, it's not possible to run regular Linux binaries that tap into certain hardware features like Vulkan without reworking and recompiling them.
Just from the top of my head, there's a bunch of other things people could name I'm sure. Android misses a lot of the expectations people have for regular GNU/Linux distributions have - expectations that non GNU/Linux distros like Alpine do meet for the most part.