Why don't prices of USA labor and Mexican labor equalise? That Mexicans workers get paid less US workers is a dynamic which motivates the moving of production to Mexico.  This idea can be abstracted to Developed Markets (DM) labor and Emerging Markets (EM) labor.  This is driving many recent political upsets.  


Do the proponents of free globalised trade expect the price of labor to converge?  
Is there evidence of convergence?
When will meaningful convergence arrive (example metric EM labor being paid no less than half DM labor)?


So folding in some feedback from commenters about 'why should they?'.  Isn't this case analogous to supernormal profits being competed away?
The underlying reason for persistent wage differences is probably that Mexican workers are less productive than American workers.
It is sufficient that workers in some tradeable sectors are more productive to have persistent wage differences in all sectors. (a good explanation of this effect can be found here: https://en.wikipedia.org/wiki/Balassa%E2%80%93Samuelson_effect#The_effect_in_more_detail)
One would expect that unit labor cost in tradeable sectors converges, not necessarily wages.
Here is a graph that suggests this has already happened: https://fred.stlouisfed.org/series/CCRETT02MXA661N, for an explanation of real effective exchange rates see here: https://en.wikipedia.org/wiki/Effective_exchange_rate