Probably because most of their logic is locked in "no code" systems, such as Unreal's blueprint or as Unity component data.
A well- and hand-written game is easily portable, see the original Doom, where separation between core logic and side effects, such as drawing in a screen, is clear. Actually, this is a good architectural model for all types of software.
An interesting example of someone who structured their game in a way that was easy to port is Caves of Qud. Supposedly he got it from Unity to Godot (after the pricing fiasco) in a pretty short amount of time.
A well- and hand-written game is easily portable, see the original Doom, where separation between core logic and side effects, such as drawing in a screen, is clear. Actually, this is a good architectural model for all types of software.