Probably because most people are incapable of documenting that their work occurred off-hours and without company resources. And even if they are, their employer

Probably because most people are incapable of documenting that their work occurred off-hours and without company resources. And even if they are, their employer can still tie them up in litigation if the company ever takes off. Working contract jobs to support your startup aspirations can make sense, but working as an FTE is pretty risky.