The thing is, with average speed cameras the level of compliance is very good. The same holds true with the spot speed kit on Smart Motorways in my experience. I set the cruise control to 70 and very few people pass me, which is most certainly not the case on other motorways.
Cameras don't generate much revenue at all in the scheme of things and that's how Highways England and HMG want it. The cost of some speed cameras in comparison to the cost to the nation for every fatality is peanuts. HMG would rather pay the peanuts for the cameras and take several decades of income tax/NI from those who would otherwise kill themselves and others, or if they survive, cost the NHS a fortune to support their injuries over the years.