Many of these game 'engines' are not engines at all in the Unreal/Unity sense of things. More a collection of libraries and code that are used to implement a game. For many games this is just fine (and sometimes a better fit than a monolithic engine).

If you can't take a read-only copy of the engine and implement a game on top of that then you don't have an engine. Marketing will still want you to name the 'engine' though!

Back in the earlier days of Unreal there was an industry joke that Unreal was great if you were making a single player third person space marine game on Xbox or PC. Unreal has come a very long way since then and is a truly wonderful achievement but it took a huge amount of time, resources and skill to get it to where it is now.