A lot of these aircraft use engines designed decades ago, which were not designed to use silencers. You can’t really retrofit them, for weight, power output and aerodynamic reasons, unfortunately. Also, a lot of the noise which reaches the ground is generated by the propeller , not the exhaust. The private flying people would love quieter aircraft, they don’t want to annoy people. Modern designs are better, but still a minority of craft.
A lot of the ‘flying round in circles’ is circuit training, where the pilot practises takeoffs and landing. And a lot of the lower level flying is because the aircraft are prevented from flying higher because of controlled airspace.
The amount of aviation fuel consumed is trivial, there are much bigger targets to go at first if you want to reduce emissions. All those school run trips, for example.
And that air ambulance (or police helicopter) pilot may very well have learnt his skills at a small airfield, so let’s hope the haters never have need of one.