I used to think the exact same thing, I own DOAX3 and I can't stop myself from playing it. The volleyball is so much fun and I really like the costume collection/customisation part of it.
No depth? Have you played it? the volleyball is actually pretty deep, the AI will fuck you over if you just mindlessly press buttons. Trust me... it's SO MUCH MORE than fanservice, actually the fanservice is literally the weakest part of the game.
Seriously, you can just play volleyball and tug of war and stuff and never even see any of the gravures, pole dancing etc. the super skimpy bikini and fetish stuff has to be unlocked after SEVERAL hours of actually playing the game.
Also, the girls will reject anything skimpy or stuff they don't like, it's nowhere near as bad or cringey as the trailers make it look. The trailers that focused on just the boobs do the game no justice at all. It's a super fun and casual vacation/volleyball sim, nothing more nothing less.