The reason why it’s because some people that make Gacha mods are teenagers and they probably don’t have that much money to put a app on iOS because iOS charges people to put new apps out there
it's mainly because of the ios security system. Not because it would charge them. You can download things through safari/chrome on ios for free, but the ios security may block it eventually. Gacha mods don't release in the app store or in the Google play store, only online, and itch doesn't charge for uploading files, so there would be no charge if it was download able online. But again, ios doesn't support gacha mods.