Selfhosted

They can't, tho. There are two reasons for this.

Geolocating with cell towers requires trilateration, and needs special hardware on the cell towers. Companies used to install this hardware for emergency services, but stopped doing so as soon as they legally could as it's very expensive. Cell towers can't do triangulation by themselves as it requires even more expensive hardware to measure angles; trilateration doesn't work without special equipment because wave propegation delays between the cellular antenna and the computers recording the signal are big enough to utterly throw off any estimate.

An additional factor in making trilateration (or even triangulation, in rural cases where they did sometimes install triangulation antenna arrays on the towers) is that, since the UMTS standard, cell chips work really hard to minimize their radio signal strength. They find the closest antenna and then reduce their power until they can just barely talk to the tower; and except in certain cases they only talk to one tower at a time. This means that, at any given point, only one tower is responsible for handling traffic for the phone, and for triangulation you need 3. In addition to saving battery power, it saves the cell companies money, because of traffic congestion: a single tower can only handle so much traffic, and they have to put in more antennas and computers if the mobile density gets too high.

The reason phones can use cellular signal to improve accuracy is because each phone can do its own triangulation, although it's still not great and can be impossible because of power attenuation (being able to see only one tower - or maybe two - at a time); this is why Google and Apple use WiFi signals to improve accuracy, and why in-phone triangulation isn't good enough: in any sufficiently dense urban or suburban environment, the combined informal of all the WiFi routers the phone can see, and the cell towers it can hear, can be enough to give a good, accurate position without having to turn on the GPS chip, obtain a satellite fix (which may be impossible indoors) and suck down power. But this is all done inside and from the phone - this isn't something cell carriers can do themselves most of the time. Your phone has to send its location out somewhere.

TL;DR: Cell carriers usually can't locate you with any real accuracy, without the help of your phone actively reporting its calculated location. This is largely because it's very expensive for carriers to install the necessary hardware to get any accuracy of more than hundreds of meters; they are loath to spend that money, and legislation requiring them to do so no longer exists, or is no longer enforced.

Source: me. I worked for several years in a company that made all of the expensive equipment - hardware and software - and sold it to The Big Three carriers in the US. We also paid lobbyists to ensure that there were laws requiring cell providers to be able to locate phones for emergency services. We sent a bunch of our people and equipment to NYC on 9/11 and helped locate phones. I have no doubt law enforcement also used the capability, but that was between the cops and the cell providers. I know companies stopped doing this because we owned all of the patents on the technology and ruthlessly and successfully prosecuted the only one or two competitors in the market, and yet we still were going out of business at the end as, one by one, cell companies found ways to argue out of buying, installing, and maintaining all of this equipment. In the end, the competitors we couldn't beat were Google and Apple, and the cell phones themselves.