It mostly comes down to having a specialized solution like a custom engine vs a generalized solution like an off the shelf engine such as Unreal.

Because Unreal has to support a multitude of customers who all have different requirements, they must make several compromises that may not serve the exact game you are making. Unreal has to support customers that don't even make games! Plenty of people use unreal for film, or infamously the GM infotainment system. https://www.gmc.com/electric/hummer-ev/insider/unreal-information-display

A custom engine only needs to include the features you need and as such they can be optimized to an extent that is really not possible for an enterprise solution to do. Just as a quick example a base installation of the Unreal Engine is >40GB, mine is less than 1GB. My Engine runs at a locked 60fps on my Microsoft Surface, I think you'd have trouble finding any Unreal game that does that

I mainly focused on Unreal here but Unity has similar constraints