As someone who actively works to get into writing(I wanna be one of those actors that directs, acts and writes shit) its kinda a easier road to drive on when you're working with pre-existing characters. They got a full background you can reference, a developed personality and a style of speaking(i.e Vader)
Writing an original story is much more tiring tbh. I've tried on the DMBE forums and it's a bloody mess. Not to mention that most original stories follow a gary stu or a self insert of the fan with the same name and no originality.(This isn't a dig at dmb, he can at the very least make fight scenes seem interesting)
To be honest, I've never seen an original story that isn't shitty in some aspect.