Because it's based on a tax % rate vs a fixed cost of $2-5k/yr per employee. So if you want to hire 4 people for 10hrs/wk paying each of them $20/hr instead of 1 person at 40hrs/wk, it's not going to cost the company an extra $6-15k / yr to get equivalent hours of labor to do so.

You need to reduce fixed costs and make them a cost that scales with the amount paid for the labor to make doing this more attractive for companies.

Other benefits are pretty much nothing compared to health insurance, or only cost money when the employees are at the job site, so govt. healthcare is needed before 4 workers at 10 hours a week becomes more attractive to employers.

Walmart & co make their employees work part time just to avoid paying mandatory benefits. Benefits are a big cost to companies.