Even if poker is a PvP kind of game the casino still has expenses to cover and since they are giving a service and they are using some of their resources to keep the poker tables online then it is natural they take a rake, and as you say this rake is lower than what we see at physical casinos, however I can understand why some may find difficult to profit from it as the number of poker players on those websites is limited and many are decent players so unless you are way above them in terms of skill then you are not getting any profits against them.
I think that you can find online casino or poker room with players who`s skill will be at least not higher then yours own. But the most interesting game is when you`re winning powerful opponents as for me. In such wins you usually getting good profit and good moon. In easy games you can get profit, but mostly it will be less then your normal profit, and surely it will not be fun.