Creating a multi-account for the sake of welcome bonuses is a pointless idea, because, as you said, almost all gambling platforms set wagering requirements for bonuses, and as a rule, these requirements are too high, which makes the pursuit of bonuses pointless both for the average user and for the avid abuser who has many accounts.
And casinos, for their part, track those who have multiple accounts and prevent rule violations, including those related to welcome bonuses. There's no point in creating many accounts that won't be used afterward, just for bonuses that can't be withdrawn in most cases. This also applies to other casino bonuses, by the way, not just welcome ones.