So here's why (and this is not particularly carrier-specific, it's the same whenever there's some new technology no matter who is deploying it): Whenever there's a new technology, whether it was GSM over TDMA, UMTS over GSM, or then LTE over UMTS, and now 5G over LTE, there's a couple of things in play on deployment speed.
One is the speed of actually being able to deploy the new technology, whether it requires new transmitters on towers, or software updates, or whatever. If new hardware is required, it takes forever to deploy it. Over a year typically from when the designs are issued to construction complete, generally. The other is the actual number of devices in the network capable of actually using the new technologies. Sure you might have the newest iPhone 14 or whatever the latest Samsung is, with all the bells and whistles, but there's just as many people out there somehow still rocking an iPhone 5.
So AT&T or CSpire or Verizon may have 5G available, but the actual demand on the network for 5G services is still far less than demand for LTE. That means the amount of radio spectrum bandwidth devoted to the new technology is still a pretty small fraction of all the bandwidth each carrier has available. And each carrier only has a finite amount of bandwidth available in each market, and they have to be smart about they divvy that out to serve their ENTIRE customer base. So if you're on the front end of things, that might suck, but you're still in the minority of users at the moment.
The good news is, as time passes and old phones get traded for new the demand for 5G will grow and spectrum will be shifted around until ultimately 5G becomes the prevailing technology, at which point you'll probably be here bitching about how your new 6G phone seems really slow.
[Disclaimer: I work in network engineering for one of the companies mentioned here, and while I'm only specifically knowledgable about one of them, I feel confident all are in the same boat, because we always have been.]