First of all, the Cowboys line is not nearly as good as it was in years past. But, your point is still valid. The pass rushers are getting better and better and OL play is dropping across the league. This is because:
OL is hard to evaluate because HS and college football is all read option or single-read spread offenses. No one has to hold a pass block.
What kid grows up dreaming of being an OL? Everybody else on the field gets to do something fun, but OL just has to block, not hold and pray not to be noticed.