In a strong labor market workers are able to demand some of the generated productivity surplus as compensation. Wages have been somewhat flat because employers

In a strong labor market workers are able to demand some of the generated productivity surplus as compensation. Wages have been somewhat flat because employers are paying workers in (untaxed) health and retirement benefits instead of (taxed) wages. Secondly, there is no particular reason wages should track productivity. Productivity is output / workers. If output increases due to capital (e.g. automation), there is no reason for wages to go up.