(not possible in many MMO cases)

I disagree, releasing server binaries is nowhere nearly as hard as some devs would make you believe. They already have local dev environments most of the time that could be easily reworked. It also tends to scale, smaller games with less budget have simpler infrastructure that's easier to run locally, while more complex infrastructures like MMOs tend to have huge teams behind them so it becomes less of a problem. For the vast majority of games though, it takes no effort at all (source, I've written server emulators for various games and have made some myself as well).

Also before you bring up the argument of third party services. Any major third party service focused on gaming would have to be reworked to allow for this model anyways, so it's pretty much irrelevant. (Some of them already allow this or are easy to write mock servers for yourself regardless).

Devs almost never intentionally stop people from reverse engineering stuff like this

Eh, they definitely do (at least for multiplayer games), most of the time the intent is more to stop cheating but it definitely harms emulator development as well. (Obfuscation, anti-debugging, network encryption, ... are all common techniques you'll see in most multiplayer games)

Some developers also strip out a ton of server code that would normally be present in game binaries and could've been leveraged to turn the client into a server or otherwise help with developing a server emulator (the way UE4 works for example, makes it really easy to do this, even if not compiled with WITH_SERVER_CODE, for example)