Anecdotal single point of evidence: I was trying something for a project at work, and between raw OpenGL, unity, and unreal, the latter two let me get a POC up and running way faster. I work with dozens of ex game industry people, and none of them have done anything from scratch for AAA. Every major studio has tons and tons of layers of engine abstractions to make development easier and faster.

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.

The issue is that Unity/Unreal are not designed from ground up for open world simulations. That’s why majority of open world games have custom engine behind them.

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.

As I understand it, this case of third party code not fitting the bill is part of the reason Star Citizen is so behind. They’re using CryEngine and have had to rip out or change a lot of the internals.

I assume with their money, they would have full license with source code.

The bigger reason is scope, fun factor and bugs.

Scope is huge, fun might be not there and bugs probably in 100 thousands.