I'm a full-time freelancer involved in freelancers' online lists, and nearly all of us are sole proprietors. You're right in that there's no one to strike against because we work for ourselves (although we do pass along tips about slow payers and other abusive clients), but there is an immense collective need for "employee" benefits, especially health insurance, that is often expressed. Many can't afford exorbitant individual policy rates for themselves and/or their families; others can't buy it at any price because of pre-existing conditions; a few have relocated to states like Pennsylvania where buying individual health insurance is feasible if not cheap.

My hobby is selling used books online, and some months I make enough to pay the utility bills. There are hundreds of thousands of online fixed-price sellers not affiliated with eBay. I'd guess most are part-time, and most have the same problems eBay sellers have, just with different venues.