Because benefits (healthcare) are so costly that the rideshare business model becomes nonviable. To that, some might claim that no business model deserves to exist if it cannot provide all of these benefits. I think that’s misguided.
The very idea that health benefits are tied to employment is the real problem. In Canada they aren’t and thus it’s far more reasonable to work as a ride share driver here. If we could go even further and add pharmacare, dental, and even basic income to the services provided by the government, then we could get rid of minimum wage laws as well. When people are truly free to walk away from any job, then the market will decide the appropriate wage. Any job offering truly unacceptable wages at that point becomes a ripe target for automation.