Dont be dense.

Corporates can pay more because they have access to other sources of capital. That's why they can keep throwing money at staff.

Professional services entities like Big4 only have access to partners' equity. Unless they're listed entities like Accenture.

You talk about increasing starting pay to 6k, but that means you have to increase all senior ranks' pay significantly across the board too, and at very great expense. Not doing so will result in "salary bunching", which will cause the more experienced staff to kpkb that fresh grads are out-earning them and start to mass quit.

Increasing staff pay across the board means hitting the partners' annual comp. Audit services cannot easily grow the top line. Unlike companies that can scale quickly and even go into deficit for rapid short term growth - see Grab, SEA etc.

Since partners are the owners in Big4, there's no incentive to account to other stakeholders except the partners. Since there's no such thing as shareholders, investors.

As long as the partners can maintain the same level of annual takehome comp, they're more than happy to continue the status quo. There's no incentive to change how things are done, when they can easily just hire more cheap jhks to replace expensive & demanding sinkies.