The same bots that buy everything in seconds would just fill the first 50K(or whatever) places in the back order queue. They already have to have different accounts, addresses, credit cards, etc. to avoid the per customer limits.
Admittedly that does give whatever company more time to investigate that queue for bot accounts. They could then remove suspicious accounts but that costs time/money and they have virtually no incentive to do it. Not to mention the backlash from false positives.
Valve is an odd duck because they aren’t a publicly traded company and thus not driven by the ‘line must always go up’ nonsense. They would be a company that absolutely would purge bot accounts from their queue. Steam account that’s only hours/days old? No games in the account? No wishlist either. No browse history of any game ever? Yeah, that’s a bot.