I don’t think it’s necessarily that something can’t be done, but each engine has its own idiosyncrasies and ways of doing things, and if you want to build something unique you’d still have to build all your own ways of doing something in another engine on top of the already existing ways of doing something. There is also a lot of flexibility in rolling your own engine, and it’s leaner too. Of course you can create an Animal Well in Unity, but out of the box it has very many systems that are unneeded, get in the way, possibly can’t be disabled and take up resources, bugs that you can’t fix and have to rely on the unity team for, etc. And if you do find bugs that you can’t work around and you use an older unity version (because migrating to newer versions could break things, and you started development 6 years ago) the bugs often won’t be fixed as your unity version isn’t supported anymore.