I apparently hate working 40-hour weeks, and I don't need as much
money as I'm making.
I'm not sure yet if I would rather stay in my current position or find
another, but I've already been warned of a huge drawback to switching
to part-time: I would lose all of my benefits. I wouldn't have the
option to use my employer's health or life insurance, or to keep
paying into retirement, and I wouldn't accrue vacation or sick days.
All of these things equate to money, and they can all be purchased outside of an employer.
Life insurance, health insurance, Retirement accounts, time off (obviously), etc. can all be purchased. You don't need to depend on your employer for any of these.
You already indicated that you "don't need as much money" as you are currently making. For the next step, simply factor in the cost for the "benefits" you wish to purchase in your decision regarding how many hours you wish to work and compare that to the hate you have for 40.
There's nothing special about "benefits". If you want to be able to purchase more of anything, you must earn more. If you are willing to purchase less, you can earn less (and presumably work less).
is there any reason that employers don't offer benefits to part-time
I know that is as much a rant as it is an actionable question, but I'll try to answer anyway.
There's no real "reason" other than lack of laws, lack of unions, and lack of competitive practice.
It's incorrect to say that "no one offers benefits for part-time employees". A few employers do offer some (usually reduced) benefits to part-time workers. And a few locales do require that some "benefits" be extended to some part-time workers.
Consider specifically seeking employers who provide a range of benefits for part-time workers. A quick Google search will find them. For example: http://www.businessnewsdaily.com/5752-part-time-jobs-with-benefits.html
Also consider that some companies still consider less than 40 hours to be "full-time". I know some companies that require only 32 hours to be considered full-time.
Benefits change with the economy, with the locale, with the domain, and with the times. In the not too distant past, many employers offered pensions. Now, outside of the public sector and academia, few do. On the other hand, employers might be getting more creative with their benefits these days - pet insurance, eye care, group legal, group purchasing power, etc. are being offered by more employers. Some are even offering education assistance and English language lessons.
It's all in the name of attracting and retaining a desirable workforce. When an industry must compete harder for a smaller pool of workers, they may choose to offer more benefits and/or better pay. If the desired workforce is plentiful, or the competition is offering less, than an employer may offer less as well.
If you are looking for full-time benefits in a part-time job, I suspect you'll have a hard time finding that. Maybe this will change in the "gig economy", maybe not.
What's the difference to the company if I'm working 40 hours or 30?
About 25% of your work output, apparently.