For the majority of America there's no such thing as negotiating wages. Do you think McDonalds or Walmart are going to let you negotiate more than a few cents here and there? Nope. The union would be more beneficial to them.
What makes you say that?
According to the first report I found via Google [1], workers who earn minimum wage make up about 2.3% of the labor force. Wouldn't that imply that almost all employees have at least some bargaining power?
Note: the report seems to only talk about workers paid hourly, I assume the statistic is no worse for full-time employees.
[1]: https://www.bls.gov/opub/reports/minimum-wage/2017/home.htm
Those people who have skills worth money like to negociate.
The great thing about that is everone can better themselves and be worth more.