As you said, it's not easy. This is the long and the short of it. Especially if you want to compress multiple pages together and somehow reasonably handle compression of toasted values across multiple rows and pages.

That said, there has been some progress in the general direction of allowing this: supporting new storage implementations, e.g. zheap (https://wiki.postgresql.org/wiki/Zheap). But it's a large effort. Consider it has to implement new crash recovery, for starters.