It's because they misguidedly care and want DXC to be a better place. For many leaving is failure. For others, they are frightened about the alternative or have spent so long at DXC/EDS/HP/CSC that they're institutionalized.
If no one complained then things would stay the same.
This is what Salvino misses - these complainers actually care about the company. The yes people don't.
For example, in ANZ we have seen no changes in the management or the culture (fear, sales led - old school IBM Sales). Some improvements in comms but beyond that it's the same old, same old. Lack of capability in growth areas. Platinum accounts slowly migrating away from legacy DXC to smaller players, pure-plays or taking in-house. Delivery is driven by lowest cost with little consideration to customer outcome, quality etc.
Nothing but talk of quarterly targets, inefficient tools and processes, incentive plans that don't get delivered until well into the year (people first, what a croc), pay review process that took too long, and became convoluted (i.e. pay rises given before salary adjustments - i.e. the company is passing of legislated rises as their generosity.
But the complainers want things to be better, they just don't have the ability to effect change because of the culture and, at least in ANZ, an underendowed, incompetent, bullying, sales led, people last culture.