Sorry but I really cannot understand how this is the case. In my short experience one can't make a significant impact to a mature large scale software product in 2 years. What kind of impact I'm talking, the kind of impact that can affect the business bottomline, that can actually transform the company. Without anyone having more than 2 to 3 years of experience the level of insight into the product would be quite shallow. Not to mention people that only stay for 2 to 3 years at a job never get to experience the long term implications of their code and decisions. They never get to learn from their mistakes since they keep making new ones and moving on