That's really irrelevant. Even if compensation was entirely in cash, they'd likely still be shareholders through index fund investing, e.g. most 401k options would invest in the company. Does that mean employees working at an S&P 500 company should never unionize because technically they're shareholders?

Employees at Google are made shareholders because its cheaper for the company than paying cash. Employees form an absolutely tiny minority. The point of unionizing is to increase the leverage of the workers at a company. Workers would benefit much more from unionization than they would from holding the stock at a company without a union.