As a former engineering manager this is obvious, in my opinion. Companies don't really like laying people off and PIPs are long and require work. If someone is underperforming, hiring a new person is easier. Alternatively, instead of retraining someone, hiring a new person is easier, and you just keep the old guys around doing whatever. Plus, once in a while they might actually offer something (they typically are very knowledgeable about past decisions)

Given how much money there is in these industries this strategy works for a long time.

The alternative is a harsher work environment that is common in other fields. Despite the popular belief on this forum, tech work has some of the most genteel management of all industries.