In places where GPS isn’t available, in an underground transit station for example, it really seems like a no brainer to just use the cell towers in range to triangulate your location.
The police can already track the location of your phone using cell towers. That’s how 911 knows where you are when you call them (AFAIK). Why isn’t it just available for the phone itself to figure out its own location so you can use it to navigate?
Actually it is, we do use both network cells and other public beacons for navigation when GPS is unavailable. It’s just not available everywhere – you need a map available of cell locations and usually this mandates open datasets for companies to use. Navigation works underground in e.g. Helsinki metro as a personal anecdote. We don’t need strict triangulation underground as cells are already so small. The metro tunnel is filled with picocells in practice (smaller than 200m coverage area cross-section).
We also use the cell network to push rough satellite locations to cellphones, in A-GPS, or more generally A-GNSSb as the same functionality is available for other systems as well. This way the phone can pinpoint the required satellites much faster, which is the main reason you can get such quick and accurate readings from your phone after starting to check your location.
Edit: AFAIK location services also enrich the information with databases of publicly visible WiFi SSIDs, using their visibility as a beacon. Scanning WiFi hotspots typically consumes less power than getting a GPS signal that’s as accurate, and is also often more reliable in urban settings and higher latitudes where the satellites aren’t as visible (though the constellations have enough satellites nowadays that this issue isn’t nearly as bad as it used to be)