Why are mask law violations not dealt with like speeding?

Firstly, penalty fines in general are not intended to act as revenue-generating mechanisms for the state. Their purpose is to deter people from committing the relevant offence and impose an appropriate level of punishment.

It is unfortunate that penalty fines for speeding are perhaps seen in this manner, but that is not their intended purpose.

Secondly, there is a risk to the police in attempting to enforce the law here. They will likely have minimal PPE and the police have a duty of care to not endanger their officers beyond what is required for the job in the ordinary course of duty. Arguably, deliberately forcing them to police an area with a high number of uncompliant individuals who may be infected with coronavirus could potentially be a breach of that duty of care.

Thirdly, it is a political and operational matter for the police. Their front-line policing numbers are reduced because of the pandemic (a number of officers are shielding, etc.) so they may simply not have the numbers to police an area as well as perform all their other usual policing duties.

It is likely that this is viewed as a low priority offence for the police — remember that their foremost duty is to protect and serve the public; they are not there to be revenue collectors for the government and many would refuse to do it if they were seen to be doing that.

We police by consent in the United Kingdom, and if the police are seen to be allegedly unfairly targeting an area to enforce compliance purely to raise money for the government coffers, that risks eroding public trust in the police and makes it far more difficult for the police to do their jobs in the future. It is something of a trade-off and I'd argue it comes down on the side of future benefits at the expense of today.