The FCC does nearly zero proactive enforcement of its rules (source: I've worked in the radio industry for nearly a decade). Violations are pretty common since there are a lot of rules to follow, though most people do operate properly or at least make a good faith effort to do so.
I’ve read that the FCC will actively pursue reports of pirate stations or other illegal interference, and they’ll drive around the neighborhood in a van to triangulate the offenders.
In fact here’s a list, but it looks like they stopped updating in 2018.
The FCC does do enforcement, its just that much of is is reactive based on complaints from other spectrum users. They aren't rolling around making sure that, for example, a license holder licensed for 10W isn't operating at 15W by using a higher gain antenna than originally specified. Those are the sort of violations that are reasonably common, but also are unlikely to impact other users significantly, so generally wont be complained about.
Anecdata from other HN threads is the FCC doesn’t really go looking for these, rather it accepts cases wrapped up in nice little bows by pissed off HAMs. Maybe there weren’t any HAMs in range or he didn’t affect their bands.
I'd like to think, for all their promises of reliability, mobile phone carriers would notice a sudden hole punched in their coverage maps, since they can monitor the signal quality of each device from their central control as they move around the area, and investigate themselves or summon the FCC once they discover it's not natural interference.
