Supply and demand.
Maybe the question is on us – why don’t we value our children’s Jewish education enough to pay their rebbeim and moros a fair wage commiserate with the value they are providing?
As far as part time=low salary, tell that to professional athletes who work half a year and get paid millions.
Instead of bashing those who may very well be the ticket to our Olam Habo, perhaps learn to appreciate the tremendous skill and dedication which the vast majority of mechanchim bring.
BTW, most mechanchim I know take summer jobs out of financial necessity, despite the fact that the intense nature of their jobs takes an emotional and physical toll well beyond the hours they spend in the classroom, and they could really use the break to recharge.