You don’t see MMO made by because networking is hard, there that have no problem coding networking like that, problem is hard to manage such project, MMO is not something you can just make and done, you need to get good hosting and establish good support of it.
Also pro tip: don’t limit yourself and wait for others to do things for you, this is something i see frequently in this community, juvenile starlings on pile of seeds screaming to be feed. Think of MMO you want to make, study a little bit about networking and protocols first and think that you can do accomplish your goal, instead of depending on others, UE4 already have some tools that may help you out. You can also try to find some programmer who is specialist in networking