Hiring 30k people so recently then firing almost half of them the next year, even filtering a tiny quarter of a percent is going to create outliers who get affected with serious life changing events.

Even if you have all the data, lets say there is a 4 person team being reduced to 3 during this redundancy - What do you do? Fire Anne instead of Peter purely because Peter joined the team more recently? Sure that sounds more fair for Peter, but it's less fair for Anne.