They don't really have the staff for it. And it can be very time consuming to do.
I had to chase down interference on one of our repeater inputs. Took quite a bit of time to do it. Lots of driving and taking bearings. Then narrowing it down, more driving, more bearings. Then lucky guess or two.
After all that, I was able to resolve the issue with the interfering system by talking with the owner like adults. Not throwing a fit, not threatening to call the FCC or law enforcement. Just explain the situation, show them the evidence, and help them fix it. Much easier that way. If I'd had to hand it off to the FCC, it's much easier if you can hand them all the details and proof. You'd be more likely to get their time to resolve it.
If the system owner starts to act like a jerk, well, then you did all you could and you hand the neatly wrapped package to the FCC and let them take over.