Because asynchronous communication takes a long time and often misses important emotional information transmitted via voice.

I am not a slow writer, but for many things I prefer to have a phone call as I find it significantly easier and quicker to clear up any potential misunderstandings.

I get that there are people that prefer text over voice and I do try to accomodate depending on preference, but if I have to have a proper conversation that may get a bit technical its simply quicker to call.