Of course you don't have to implement it to the exclusion of all others! But let's suppose that, say, 65% of users have a Google account, 75% have a Facebook account, and 40% have an Apple account - and the circles overlap such that adding Apple adds 10% to the total coverage. Now, if everyone who has an Apple account prefers it, and users with an Apple login yield 30% less revenue... gaining that extra 10% of users costs you revenue, because for every user you gained at 70%, three existing users cut their revenue by 30%.