The social order is fundamentally nepotistic. Family/Tribe/Caste matters more than experience/performance within the managerial bubble. Keeping the actual workers down helps to keep them desperate to put food on the table, but you better perform well enough to float the non-workers. Deloitte/Wipro/Infosys/et.al know how to play this game very well to discourage anyone not in the tribe from advancing.
One of the reasons I have realized is that it's sometimes about control. A manager (people, project, product) will not have as much control over an experienced employee who has potentially been at the company a long time or just has a lot of experience in general in terms of getting what they want from the employee, despite the employee having the business' best interest in mind. So it's an implicit attrition that seems to sometimes be desired.
