What you seem to be saying that if 100% of employers treated their employees well, there would be no need for unions.
The fact is that in many workplaces, there's already no need for unions. But the landscape as it exists has most people believing that you need unions, and even the employees who don't want a union feel peer pressure to vote for a union. I still remember seeing a sign showing the union vote - 99 to 1 for having a union. Next to the "1", some had written some choice language describing the individual.
So everyone votes for and goes along with the union, even if they don't really want it. I've had employees who we were forced by the union to fire because they didn't want to be a part of the union and didn't want the union taking their money.