How? Staff an OSHA inspector to stand around in every garage? If Jim at Jim's Auto Service doesn't want to wear a mask, he's not going to wear one. It's not like oil where there's an evidence trail and a big mess if he decides to dump 50 gallons in the back lot.
Yes, there's more that could be done in regards to PPE enforcement, but I think that's really an orthogonal issue. Asbestos isn't necessary in brakes, and can be banned regardless of PPE enforcement.
The same way they manage to conduct health inspections even in the most random hole in the wall greasy spoon locations. You have an expectation for certain things to be there. You have an expectation that procedures are followed. When an incident occurs or an inspection happens where its clear that certain protections were not in place or certain procedures were not followed, you drop the hammer on them. Its not hard and we already have the bureaucratic apparatus to do this. Saying we can't do it for mechanics while we simultaneously do it for other industries like farming, construction, manufacturing, or food service, is just lazy.
What you are describing is exactly how it is already enforced today. But an "incident" regarding asbestos (or other particle) inhalation might not appear until decades after exposure. Many tradespeople, automotive or otherwise, use the amount of PPE they personally are comfortable with. Construction is notoriously similar. You will see more compliance at larger organizations where there is more governance, but at smaller orgs, people often do as they please.
Yes, there's more that could be done in regards to PPE enforcement, but I think that's really an orthogonal issue. Asbestos isn't necessary in brakes, and can be banned regardless of PPE enforcement.