Because the gig economy is hard if you have to pay union dues, have a union control how your work, etc.
"I can't afford health insurance, but it's better than paying MUH DUES" –every Uber driver ever, apparently
1. Oftentimes people don't take into account the long term cost/benefits with each job.
2. Depends on the union. Some unions have exclusive pool plans that were grandfathered in under Obamacare, meaning that there's no maximum cap on out-of-pocket costs (so a $1,000,000 hospital stay (reasonable for a car crash, for instance, can't believe I actually had to say those words) could cost $100,000 instead of hitting a cap of, say, $8,000). Our health care system is sh**t, so no solution would work that well.
But the biggest thing: It isn't a full time guaranteed (i.e. 9-5) job, meaning that if a union says they'll go on strike there won't really be many issues compared to, say, a steel plant.