"Are You Open Today?"

I know I've been talking about appointments lately, but I keep getting phone calls that have the same sort of idea to them.... and I don't quite get it.  Maybe someone can enlighten me.

Someone will call, and ask, "are you open today?" or "what's your hours today?"

My response is -- we do everything by appointment (since we are not like a retail store in a strip mall or whatever)... and I tell them, they CAN come that day, but they would just need to make an appointment.

Every single one of them responds, "ok," and hangs up.

I don't have hours, because some weeks I have no customers, like AT ALL, all week, and other days, I have appointments from 10 am until 9 pm.  For the not-steady flow of business, it's not practical to have hours. 

But I guess what I don't get... what is so hard about setting up an appointment?  I'm not saying it has to be like a freaking doctors appointment (and these people don't seem to think so either, as at least half show up late), but the second I say the word "appointment," op, nope, sorry, they vanish.  I get that it's more convenient to show up whenever, during a set frame of hours... but... I guess setting an appointment doesn't seem that difficult to me?  Anyone care to explain?