Well, the thing is that MOB As
happen on a 3D plane, in which you can kinda move anywhere horizontally, and only horizontally, as you usually don't jump. Most fighting games, you're on a 2D plane, where you're moving either forward or backwards, and the vertical plane is much more important(Even more so in Melty Blood or games where you double jump). Some has you switching between planes by stepping sideways(Jojo ASB comes to mind), but overall, the basis is still moving forward and backwards on a 2D plane. That's going to be hard to transition between. Say, Ryu's Shoryuken, that has very good vertical range, but bad horizontal range, is going to be almost useless in a MOBA, as a MOBA places much less emphasis, even none, on vertical movement, and you don't even have to step backwards to avoid it, you can go left, right, or diagonal movement to avoid the Shoryuken. Or perhaps, Ashe's ultimate, which is a super-long arrow, is very useful in a MOBA's very wide field, but in a fighting game, where fields are fundamentally much smaller, it's not going to be anywhere near as effective, and then you could just jump to avoid it, or block it as you can see it coming.
I can see your point, but when we actually try to transition from one to another, it becomes very hard. I'd be interested in seeing how people might think of creative ideas to do that though.