Because of the asymmetry in laboral relationships.
If you have a rare, valuable skill, employers will be willing to pay handsomely for your time, but they will demand a lot of your time. IF you are good enough, they will be even willing to pay you premium to have you not use your skills for anyone else. The caveat is that these arrangements are, more often than not, non-negotiable. It is an all or nothing proposition.
On the other hand, if your skills are commodity, potential employers will be happy to hire you part time (so they do not have to give you any benefits). However, your wage will be so low that, more often than not, you'll want to juggle two or more part-time jobs so you can earn a reasonable compensation that actually pays for your living expenses.
If you have a rare, valuable skill, employers will be willing to pay handsomely for your time, but they will demand a lot of your time. IF you are good enough, they will be even willing to pay you premium to have you not use your skills for anyone else. The caveat is that these arrangements are, more often than not, non-negotiable. It is an all or nothing proposition.
On the other hand, if your skills are commodity, potential employers will be happy to hire you part time (so they do not have to give you any benefits). However, your wage will be so low that, more often than not, you'll want to juggle two or more part-time jobs so you can earn a reasonable compensation that actually pays for your living expenses.