But sometimes people create a second account or an account for some odd reason that isn't directly obviously them. But they'll properly use those accounts. Do you just ban these accounts too? Accounts not doing any harm and potentially being beneficial to certain things like FB group participation or posts.
Finally, if they act like real users since they are, it helps Facebook's bottom line. Booting them would hurt FB and piss off those people.
They could handle that use in a few ways. The easiest would be to tie the second account back to the first, so that FB knows but nobody else does. But presumably people are doing that for a reason, and the real solution is to create a feature for one's main account that accommodates that reason. And banning is not an unreasonable solution. Facebook is demonstrably ok with losing small amounts of marginal traffic, especially if that helps limit abuse. Their number one asset is trust, and anything that chips away at that trust is extremely dangerous for them.
Maybe people don't want the two accounts associated? Or then they might accidentally post in the wrong one? That can already happen but the blame lies entirely on the user right now.
I do agree there are likely solutions like you're saying. Pages already work that way. Linked to a personal profile. Multiple can be.
Though I guess it likely isn't in Facebook's interest to do a feature like you propose if they can help it. If they can ban enough actual worthless or hurting fake accounts without messing with fake accounts that are being used legitimately enough. Since that boosts their numbers.
Finally, if they act like real users since they are, it helps Facebook's bottom line. Booting them would hurt FB and piss off those people.
This is just one example of "fake accounts"