I think it’s assumed that in a society that would allow for such things benefits wouldn’t be handled at the level of the employer.

Quite the opposite. Employees under 28 hours a week don’t have to be provided health benefits at all, and typically aren’t in industries like retail that mostly employ front line workers on a part time basis.

Part time help is cheap.

Those might not be proportial, disregarding fixed costs, on some places those costs/deductions are usually based on a percentage of the given salary or hours worked (Not so in the US, as I'm given to understan)

I was so surprised when I realized stat holidays were paid as a % of the previous 30 days salary in canada.

Productivity per hour increases though, so it compensates.

I could see it going down if you do something like more work or work like hobbies in the other time instead of more leisure.

That is entirely independent of working shorter hours though.

I'm a contractor because I work remotely for a foreign company, and they denied me paid holidays with this argument. The cost of vacation is proportional to worked hours. There's a lot of questionable math in HR land though.

I could see maybe the argument against this because of paid accounts in remote systems: google suite, office 365, bamboo, github, etc. compared to reduced use, but they set those up anyway for other people in the company who don't use them (non-devs, etc) and I don't believe the cost per user is significant.

Do you mean tax costs? AFAIK typically country requirements for benefits are proportional to time worked, so part timers don't get all the benefits. Which benefits are you talking about specifically?