I remember Clover-TAC saying the exact same thing in an interview. Something like "In RPGs your character gets better, in STGs you get better". Problem is, the progress isn't guaranteed; in an RPG you can be sure you'll hit the next level sooner or later, in an STG you might never reach the end of the game (or a high score or whatever your goal is). Also, STGs usually don't give you bars to fill, unlocks or other types of rewards to measure your progress and make you feel better. That's what makes them more intimidating than RPGs, imho.