Because you can imagine the incentives if a company had to pay for health insurance for a part-time worker who works 10 hr per week?

All those part time jobs would disappear since what kind of employer pay $5-10k per year in insurance premium for an employee who earns $600 per month before taxes?

And you might say “great! Who cares about part time work?” and you’d be wrong, plenty of people don’t need or can do full time work.