And if you're a recruiter and you can't listen to a request like that, how are you going to listen to the rest of what the candidate says?
It is not trivial either. Neither side wants to show their full availability immediately and you eventually have to block multiple timeslots till a response comes in. On a call you can give out preferred times at first and if they aren't a match look for alternatives. And you can confirm on both calendars at once without blocking other slots as well. But yes, shouldn't be hard to manage ...
I've only had that go more than a couple rounds when trying to setup meetings with someone incredibly busy (and those people often have assistants that I can work directly with to find a slot). I'm not opposed to getting on a phone call to setup a meeting, but only after the vastly more efficient method has failed twice.