YouTube sets the rates it pays creators not the advertisers. A union would require YouTube doesn't arbitrarily change the rates it pays it's creators and also might be able to prevent them from instantly demonetizing videos. A union would create stability in the marketplace.
For example, if you're contracted to do a job, the client doesn't usually arbitrarily change the rates it's paying you unless you have some contractual agreement to do so(overtime, paid leave, etc.).
But that's built into the original contract during negotiations.
YouTube is just "here read this shit and do what we say and we can pay you or choose not to pay you or change what we pay you whenever we feel like it whether you like it or not"
For example, if you're contracted to do a job, the client doesn't usually arbitrarily change the rates it's paying you unless you have some contractual agreement to do so(overtime, paid leave, etc.).
But that's built into the original contract during negotiations.
YouTube is just "here read this shit and do what we say and we can pay you or choose not to pay you or change what we pay you whenever we feel like it whether you like it or not"