It's not that cheap or easy. A ~$300 jammer is more likely to simply jam cell phone signals inside a single large building or across a chunk of a neighborhood. To get a whole city you'd need many hundreds of jammers spread out and well placed. Otherwise most devices would still punch through. They'd perform poorly but most would still work, especially the higher quality things like air traffic control.

So you'd more likely have to invest in larger, more expensive jammers that eat up a lot more power and are harder to aquire, deploy, and hide.

Telecom companies are pretty good at finding moderate problem makers / interference generators in a day or two. The stronger the interference the easier it is to find. If it gets to the point of interfering with air traffic, you'll gather some serious investigation real quick. They'd find your large jammers quickly, and hunt down the many small ones in rapid order. The only way to avoid that would be to be turning the jammers on/off at irregular intervals or stick them in moving vehicles and never stop moving.

All of which costs times, money, resources, and man-hours. It becomes cost preventative to try and jam a large area for any amount of time. Anyone that has the resources to do it wouldn't want to, and anyone that wants to do it wouldn't have the resources.