The companies of 2018 are playing chicken with the labor market. They are not offering higher wages, betting that the workers will start to get hungry long before the stockholders start to notice that the cabana boy has been delivering smaller drinks to their beach chairs, at longer intervals.

They don't have to pay more. The stockholders just don't know of any place better to put their money right now. It's not like a startup could undercut and take all the business by choosing to funnel less profit towards the founders and investors. Investors won't invest in a risky venture unless you dangle a good rate of return in front of their faces. Established companies won't change course from a proven business plan.

If you pitch to investors a grocery store that will operate on a 1% profit margin instead of 2%, and give the difference to the store employees, in the form of living wages and solid benefits, you'll get laughed right out of the boardroom. If you instead walk in and say that the workers have cartelized, such that no existing store will have any employees unless it offers living wages and solid benefits, you're far more likely to get them.

... they aren't paying workers more though, because there's no unions around to give the workers enough bargaining power. This is why unions are needed -- a random worker needs his job more than the company needs him, and is not on equal bargaining footage.

If your argument actually worked, then they would be paying workers more. But they aren't, so it doesn't work, and unions are needed.