Because the gig economy is hard if you have to pay union dues, have a union control how your work, etc.
I don't exactly know how much union dues are in the US; but I can say that here the biggest labour union (CCOO) charges 14.5€ a month in union fees. The 2nd largest union (UGT) charges 13€ a month, but with low income workers (defined as up to 1.5 times minimum wage) paying only 9.5€ a month.
Those are not exactly some outrageously expensive numbers? Are US union dues into the hundreds of dollars?