[JoshuaEN]

Because the initial "hi" forces a context switch which the recipient (after reading and replying) has to either: Sit idle while the sender writes their actual question, or try to context switch back for a tiny amount of time.

10:30:01 [Coworker] hi

10:30:12 [Me] hi

10:30:35 [Coworker] do you have time for a call?

10:30:39 [Me] sure

Versus:

10:30:01 [Coworker] hi, do you have time for a call?

10:30:16 [Me] sure

This example isn't really that bad, but it is showing basically the best case with a simple question. It gets a lot worse if the sender actually has to type out a long message, or if there's a gap between each response because the other person was busy at the time.