That's not entirely true.
Most PostmarketOS devices start out using LineageOS kernels, and many are atill using those.
Why not use PostmarketOS kernels on LineageOS?
The ultimate goals are different, but cooperation on upstreaming kernel work would benefit both.
LineageOS kernels are AOSP downstream kernels, and PostmarketOS has expressly deprecated their use. LineageOS is now working on running their system on close-to-mainline kernels, as provided by PostmarketOS and most Linux distributions.
> Mobile software is unfortunately not really a lego that can always be combined at will.
If we're talking about the mainline Linux, then it this looks exactly like a Lego to me. I hope that FSF will concentrate their efforts on that.
In your examples you compare Android rebuilds with real Linux distros. The projects also have quite different goals (providing full manufacturer ROM replacement for Android on Lineage OS to reusing any old hardware to basically run servers on PostmarketOS).