Why does a corporation making more money result in its employees earning more? Out of the goodness of the CEO's heart? I don't believe in this trickling down. They obviously need to pay more in order to attract the top talent, but why?
An employee costs you X per year, so you need to make a multiple of X to justify the hire. The higher that multiple is, the easier it is to make that decision. When there are lots of companies competing to hire tech workers for easy money, salaries rise.
Because they think they need the top talent, and the top talent is willing to work elsewhere where more money is on offer.