Yes, I know, – I think I thought so too.
But without a cave the fish actually has nothing to ‘defend’ and might be just stressed, I quess.
With the cave he is positively defending something which obviously is important. And it also of course has to do with that the male now can retreat into his ‘home’ and relax, feeling good – and most of the times the fishes just stand there in the can 🙂