SKW explored the idea of part-time pilot lines at one point, but I think the sticking point was the cost of initial and recurrent training.

Nobody would do this job for free for very long, so the cost of training newbies would outweigh the benefit.

Also airlines don't have much control over part-timers who earn their living elsewhere. They would ignore annoying company directives, some of which might impact safety.

Obviously no pilot group would sit still for pilots who get paid less than their contract rate...

And the first fatal crash involving "hobbyist" airline pilots would bankrupt any airline after the lawyers got through with them