Perhaps a combination of many things.
1. Why should they bother � hanging around the precinct, or playing X-box is far more entertaining.
2. Rates of pay � how much to wash a car? Give them a fiver they�ll expect a tenner. A fiver doesn�t get you much, so is it really worth the effort? Would have to do 3 or 4 cars to make it worthwhile
3. The stranger issue that you have mentioned, and the strangers suspicion of children (aka hoodlums, potential thieves, vandals etc) hovering around their house and car
4. Time, kids have a lot more homework and social lives these days, maybe things will improve in the summer months when we have longer lighter evenings.
5. Do the parents want their cherubs doing odd tedious jobs � most would rather their little darlings were project managing, than doing any manual labour!
Having said that, I have had a couple of the local kids offering to wash my car and had some group or other doing some odd jobs for charity/fund raising, so it does go on. Unfortunately, our cars are quite expensive so I declined the offer! But if they wanted to wash the drive, or clear some snow for a few quid etc etc I would never deprive them of their entrepreneurial ambitions.
Have you tried talking to your neighbouring parents?