To be fair, it’s really hard to get a hold of RPGs and .50 machine guns. It’s also really hard to hide them. And most damning of all, it’d be suicide to use them.
Most criminal firearm use is not directed at the police, but at civilians. Most criminals are out to exploit weakness, not to wage war on the strong. They want to extract the greatest possible value out of people for the least possible amount of hassle. Killing cops is entirely antithetical to criminal enterprise.
You can rob, grift, cheat, and steal from the unwashed, ignorant masses and the police will do the bare minimum about it, especially if no one actually ends up hurt. But nothing gets the attention of the police like a cop killer. They will hunt you down out of principle. They will pull out all the stops. They do not let it slide, ever.
They turn a blind eye to lots of petty crime that isn’t “worth the effort” of stopping (or often even possible to stop), but when you threaten them and theirs, when you challenge their authority, when you mess with their bread and butter, expect the hammer to come down hard and fast.