The thing that always gets me is people advertising game server engines using pretty in-game footage. That's not the server, that's CryEngine or whatever.
The people paid to create these promotions could build the same shit out of absolutely nothing, completely from scratch. They are all highly experienced and don't need to lean on either platform. It's all a ruse, and perusing projects built with either "engine" (which neither of them are by any stretch of the historical definition) show how much leverage they truly provide developers.
You'd just as easily find that you could hammer a nail with your wallet as you'd find yourself able to quickly build a prototype with either sorry excuse for software.
An apt analogy in the service world is that when I want to write a service, regardless of how many years experience I have, I don't implement my own load balancer. Could I? Yes. Should I? Only if my project is a load balancer.
Build what you need. Buy/ participate in OSS for the rest.
A lot of engine parts are off the shelf solutions for physics, animation, production tools etc,but they are all put together in a specific way for a specific project.
Also if you use 3rd party software in games it creates huge risk, especially when you don’t have access to source code or support is bad.
So in the end of the day a lot things are written from scratch or have an extremely custom integration.
The bigger reason is scope, fun factor and bugs.
Scope is huge, fun might be not there and bugs probably in 100 thousands.