And thus reduce the necessary buy in, both for players and for DMs. It strongly limits the interactions available.
This is just me, but I find the idea of sitting down at a fresh table with an already generated character to be...wrong. Unless it's organized play (official or not), where there are hard-and-fast rules already in place. To me, character creation and the associated rules happen at session 0, and they're mainly dictated by the setting and the proposed campaign. So "legal build character"(s) just don't exist until there's a table to play at. I guess I don't expect that everything's available until told otherwise; I assume that I'll be told what exists in the world/setting/campaign and anything else just isn't there to be chosen from. To each their own.