That does seem counter-intuitive ... although can there be Steam links (other than in sigs)? If not, that would be the only thing that makes sense ... that F95 won't link to direct purchase sites, since Patreon/SS is subscription and Itch.io is "donation" and Discord/Musk-laden site is promotion ???
Oppaiman isn't a store per se. When you "buy" a game, you don't pay them. You purchase a monthly subscription to Patreon (the dev specifies which Tier you should subscribe to get the game through Oppaiman).
In my case, even if Season 1 is available to every tier, I put it at 10€ (Emma's tier) to be coherent with the price I will put it on Steam (if I ever publish there).
Obviously, since Oppaiman "forces" the player to subscribe to my Patreon, they have access to Season 2 and the first chapter of Season 3 in a single payment, but I guess I'm a special case and every dev publishes there the current update instead of one Season from a year ago.
I could guess why Oppaiman is
Damnatio memoriae on F95. That page belongs to the developer of a game known for its "Fresh Girls" (the girls could have grown up) and we know some devs are at odds with F95 staff (or, more precisely, the staff is angry at some devs by whatever reason).