DOA is not a hard game to grasp at all, and so many people struggle in the game because they over think it way too much.
I think one of the big problems other "fighting game fans" have with DOA, well DOA5 in particular, is that they underestimate how in-depth the combat system is and try to play it like every other 3D fighter. How the game is played online is not only wrong, it's irrelevant because the lag essentially prevents the game from being played properly.
For instance, there are three levels of stuns that lead to 3 different launch heights. You need to decide whether you'll try to reach Stun Level 3 (Critical Stun) and go for a critical burst or a max height launch, or launch earlier at the expense of a lower launch height, or go for a hi-counter throw in anticipation of a counter hold attempt.
All 3 launch heights require different combos to be learned to maximize their launch potential. This is further compounded by the multitude of different weight classes, most notably with trying to juggle Bass or Bayman.
Oki is also an area of challenge to develop in DOA in the form of Force Techs in order to maintain offense. It's not enough to just get a max damage juggle, but you have to think about how you're going to maintain offensive pressure and keep up the attack. Setting up Force Techs with some characters is not always easy.
There are also combos that can be performed off of environmental objects, walls, or ceilings. There's a lot of things that have to be taken into account when playing the game. If you're playing this game and mashing buttons offline, you won't get very far. I mash buttons when I play Tekken online and I do alright, but I know that it wouldn't mean shit offline.