A lot of my colleagues and ex colleagues either work or have worked in Infosys/Wipro/TCS at some point in their careers. And again, they didn't "magically" get worse or better when they left or joined my company (I'm in a product development company).
Wipro, Infy etc aren't able to hold quality senior technical talent because they cannot pay as much. The better senior developers leave to product development firms that pay better and have better quality work and work life balance, leaving behind newly joined junior developers who know less, but will accept what Infy/Wipro pays and work ungodly hours.
I've read that every 5 years the number of active developers doubles. That means that every 5 years 50% of the development workforce has little to no experience. In India, that 50% starter workforce is concentrated, sitting in the big consulting firms, developing their skills and getting better, and when and if they get better, they leave.
The real problem is that if you pay peanuts, you get... people willing to work for peanuts, until they can get a better job.