Why the heck aren't there apps that warn you when a new cell tower pops up in an area?
There's AIMSICD, although I'm not sure how accurate it is. I played with it a bit last year and got a few yellow warnings, so the app detects something, but it's possible those were due to legitimate roaming or tower-sharing mechanisms. When protests were ongoing in Baltimore last year, multiple people with the app reported seeing orange warnings, which mean there's definitely some fuckery going on nearby, and red warnings, which mean the user's specific phone is being targeted.
My big problems with AIMSICD last year were that it chewed through battery, the cell tower map never worked right, the upload function for OpenCellID.org was hit or miss, and there was little or no proper documentation about what the app actually does or what its different indicators mean. I think English was a second language for (most of) the developers as well as many of the users; this made the wiki and issue tracker difficult to parse. As neither an Android developer nor a subject matter expert, there wasn't much I could do to understand what was happening under the hood.
That aside, the trouble with any stingray detector app is that it runs at the consumer OS level and can only know the "facts" that OS chooses to expose. It seems likely to me that most of the unconstitutional warrantless wiretapping functionality would operate at the baseband level, below and perhaps invisible to Android/iOS/etc and any apps running there.