Because it's a very uneven power balance. Big company using young inexperienced workers that just want a foot in the door on a childhood dream and have no idea how much their work is worth. Most companies knows to exploit this to the full. Especially known in arts, music and game industry. Less so with (metal) industry jobs where there are old unions helping the workers to a healthier power balance. Except for Musk that seems to be a strange new coin behaving like the industry did 200 years ago.