Not at all.
Server CPU's aren't consumer CPU's and so new/used pricing is totally different. They're meant for professional use. If some company decide for new server, they don't look in used market. So, the only buyers for used (outdated) server CPU's are end consumers like you and me. But we're usually not ready to pay much above, say, $1000 for it -one need to have some money for expensive motherboard too.. and RAM... PSU...
Or let me put it that way: CPU's like Xeon Platinum 8176 were overpriced from begin with (no real competition at that time). Right now, one can get better CPU from AMD for less than a half of that price (i.e. Threadripper Pro 3975WX ).
To answer your question: they're not useless -if price is right and you wish/need to build a "real" server. The question is, how many of us need server at home? Exactly! -and that's why old server CPU's are kinda cheap.
a large server farm is limited by how much work each cpu can do for the users. a newer faster cpu with likely more cores can do more work, thus they upgrade.
the used stuff is not gonna be sold to some other large company who also needs the fastest they can get, so it gets passed down to those like us. they are used, old models and not wanted by large companies. all that together equals real cheap for us down the line
they are by far not useless, but if you only have room for 100 cpu's in your server farm, the 100 fastest/most cores you can get is the way to go.
Servers like the ones $8K CPU's go into are part of large server farms that represent a huge investment intended to make money for a company and it's customers. When you put that much money into a project you'll want some pretty good assurances it's going to work so you tend to avoid sketchy hardware. Buying only new CPU's with warranty and paying Intel's price for them (with negotiated discounts from list and kick-backs) is a way to make sure you maintain up-time metrics. And give yourself a way to spread the blame around if something goes wrong. Think of it as job security for CIO's who're cautious by nature.
But for lowly IT techs putting together an enterprise class server in their basement to hone essential SysOpt skills in safety...wow. Such a deal!