Because they're not equally important. Most engineers don't add billions of dollars of value. If a miscellaneous engineer resigns, it will have in most cases little impact on the prospects of the company. Most add only incremental value. The superstar seven figure ICs do get paid as such but they're rare, most people aren't that capable of that much value add as an IC.

The company made billions in profits. That could be due to a number of reasons. IP, relationships, network effects, scale economies, regulatory capture, brand equity, etc.