They did it for the DC10 and 787 (grounded for inspections). They didn’t need to do it before because Boeing was an actual engineering company
To be fair, there are a lot of rose-colored sunglasses in the crowd of Boeing critics. The rudder problems with the original 737 weren't exactly McDonnell-Douglas's fault: https://en.wikipedia.org/wiki/Boeing_737_rudder_issues
Especially since the two are so tightly intertwined.