Why the minimum wage must always be too minimal

It came to me, while eating at McDonald's, that the minimum wage can never be pushed higher than highest labor costs businesses can pay -- like McDonald's.  Meaning the minimum wage can never squeeze out all the pay that lower labor costs businesses can possibly pay -- like Walgreen's, Target or especially Walmart.  Like almost everywhere else.

McDonald's has 25% labor costs;
Walgreen's and Target have 15% labor costs;
Walmart has only 7% labor costs.

If Ronald can pay $15 an hour, then;
big retail stores could pay $20 an hour;
and super efficient Walmart should be able to pay $25 an hour.

Unions are the only way back and here is the only way back to unions that I've ever heard of:
Why Not Hold Union Representation Elections on a Regular Schedule?
Andrew Strom — November 1st, 2017
“Republicans in Congress have already proposed a bill that would require a new election in each [private employer] unionized bargaining unit whenever, through turnover, expansion, or merger, a unit experiences at least 50 percent turnover. While no union would be happy about expending limited resources on regular retention elections, I think it would be hard to turn down a trade that would allow the 93% of workers who are unrepresented to have a chance to opt for unionization on a regular schedule.”
https://onlabor.org/why-not-hold-union-representation-elections-on-a-regular-schedule/