Again, this all comes out to- easy to do out of the engine in a 3d program.
not really possible in engine only.
If you work outside the engine, you rarely have any interest in making accommodations for one of the smallest market shares out there (ue4). You target the ones people actually download a lot (unity).
Thats basically the same with Make Human anyway.
they target all platforms rather than just unreal.
probably the same for daz and definitely the same of mixamo
also, you probably won’t pass marketplace validation unless you sell a full working character. So you can’t do something like buy this to add a coat on this character series which essentially only contains a coat.
You as an artist could maybe setup up a digital shop, Which isn’t the marketplace, where people could buy garments and the character template. Pre rigged to UE4.
But like you mentioned its not cheap and super fast to develop either a system that works, and you’d be hard pressed to find clients because people looking for modular characters pay a team to make them.