There is still an element of unknown because both parties do not know each others numbers, which allows employees to still negotiate. You are now talking about information asymmetry where the party with the information will now have all the bargaining power.
When I went from working a $150K job to getting offers from Meta at $300K, the initial number they offered was $250K, and we worked upwards. I absolutely would’ve taken the job even if they offered $200K and not negotiated. But they did, based on information asymmetry. Now imagine a world where meta knows exactly how much I make and all the other information about me. I’d probably get a minor bump over my previous salary.
Edit: I ended up taking a different offer. I don’t work for and have never worked for Meta.