Funny as I had a talk about this with tripstone a few weeks ago.
Well first things first, they are completely different skills, although having one may help you with the other. In Singleplayer, the goal is to make a snazzy replay with nice flow and things that make people drop their jaws in awe with no set time limit.
In Multiplayer, the goal is to win your match. You don't look at how nice your tori is moving, because that doesn't matter in multiplayer, what matters is quick thinking and sometimes luck.
Singleplayer takes skill as you have to know how to move properly which you cannot learn in multiplayer, and you have to be patient as hell if you want to make those perfect hits.
Belts have nothing to do with singleplayer skill, and some people just learn easier than others in singleplayer. Its like school, there are those who can pick up on the concept very quickly then there are those such as myself who have been playing for years but can't make a decent replay to save my life.
Also when you say "he just stands there" you make it sound like that he is a sitting duck, and while he may be in some cases, you have to remember to make a great looking replay you have to actually get to him fluidly and smoothly. Take manips for example, uke is just standng there, but you need to have to be able to transfer flow properly so your hits will actually push him and you can manip him. If you can't transfer flow you won't be pushing shit.
tl;dr - Singleplayer is much harder than you make it sound, and although you believe that it takes no skill and anyone could do it, not many people have the patience to get those double cores, or seven hit boomskeets. I'd like to see you try and make a replay of ormo quality since you are way past blue belt and see how far you get.