Ask HN: Why does the Signal Protocol use so many keys?

After reading through a cursory summary of the signal protocol[1], I'm wondering why it uses so many keys? I understand the sentiment behind using various keys (i.e., for sessions, chaining, short-term/long-term keys) but does any of this feel like a bit overkill?

[1] https://security.stackexchange.com/questions/121304/how-does-the-signal-protocol-used-by-whatsapp-work