Blackshift - I understand your views and share the sentiment
You would think that a flying instructor could make a similar income as a driving instructor and that would be great. I suspect that the reason we can't is for some or all of the following:
1 - The weather !
2 - Not as much demand
3 - Less competition
4 - More stringent regulations and licensing / medical costs
5 - More expensive to qualify
6- Exploitative attitude due to quick turnover of instructors
Sad but true !