The success of MUGEN isn't by complete accident. The low res simplicity of the sprites and is easy enough for common people to create in their freetime. This isn't ment to downtalk anyones talent, because it is still shit ton of work to do these mugen nude mods. But it is still easier than making a whole game in 3d in the same quality.
Alot of source film maker movies are not games! And they are still a lot of work, if you want nice quality. Still msot source film maker movies are pretty bad compared to games renders and the models aren't created from a scratch. The modded parts themself often look bad (vagina, breasts and breast bounce) because it is not easy to do. Many mugen mods look more appealing and have much more animation because they are low res pixel art and faster and more linear to do. Rotating fingers, hands, limps in 3d is alot of work, and even more so for throw animations, when they have to perfectly grab a body part. They are not allowed to slide or clip through the parts they are holding. 3D is a shitload of work especially in pron.
And again, GAMES are always a shitload of work, which is something some people here don't understand. BUT pixel art is on the simple end of the spectrum.