Actually, personally hiring an agent seems like a GREAT idea, and these things exist in all jobs, even if it's not common. Your incentives are quite closely aligned.
Having a collective agent is almost the polar opposite.
Can you explain how is it almost the polar opposite?
Things like retirement and health benefits are not negotiated at the individual level. having a personal representative likely wouldn’t help much in that area, but a union would.
Sure, maybe "polar opposite" was a bit much, but there's a big difference between negotiating for a collective set of shared benefits vs specifically negotiating for your interests. Of course, your personal agent has other clients, but is not only negotiating for precisely what you ask for (of course you will not get everything you want!), but is also likely not even negotiating on behalf of anyone else against the same organization!
In the event that your personal agent is negotiating for you personally, but also for several teammates, of course there is then a conflict and your interests may be traded for the interests of another one of his clients, which is an issue.
This is still better than having your interests pooled together, but not as good as having ONLY your interests advocated for.
It's far from certain that your interests get traded for someone else's; just as likely the opposite. Further, on some things you are in a much weaker position individually than you are collectively.
It's not objectively better to negotiate by yourself, stop pretending like it is.
Things like retirement and health benefits are not negotiated at the individual level. having a personal representative likely wouldn’t help much in that area, but a union would.