Simply because audit services are hard to differentiate between the big4
If you increase salary you have to increase the charge rate to clients. Once u increase charge rates to clients, clients go for another big4 firm which can charge lower since no differentiated services anyway.

Big4 firms need to collude and increase the charge rate together before they can increase salaries of auditors. But even then, they will lose to boutique firms which can charge lesser due to them being more cheapo with salaries.

As you can tell by now; its a race to the bottom. See who is willing to charge lesser and hence have more business. So pay will never increase. You see HR like in EY resorting to stupid reasons in order to attract fresh grads, reasons like
1. Young workplace (of course this is gonna be young because experienced people would never want to work long hours for that low pay right? So they have to dupe new blood to join the firm everytime)
2. Work anywhere (if you are on project of course you have to work where your project is at. This counts as anywhere technically right?)
3. Pantry (pantry restock at most $200 per week. Cheaper than increasing pay for auditors lol)

I mean come on..