Staggering Incompetency

Why is it, I maintain in repeating to myself, that people who do the same thing every day display such staggering incompetency in their basic task? I called my local Car shop to arrange My First Annual Checkup. The conversation went something as follows:

ME: Hello, I'd like to arrange an annual service please.
Person: Yes, certainly.

[awkward silence]

ME: Well, how is tomorrow afternoon?
Person: No, can't possibly do that, what on earth are you asking me to do that for? The earliest possible time is on Friday.

[In which case, why not damn well say so to begin with?]

ME: OK, how is next Tuesday?
Person: Yes, fine.

[Another awkward silence]

ME: So, shall we say, 3pm?
Person: No, that's far too late, you fool, it'll take much longer than that.

[Again, why not tell me what times will work, rather than making me guess]

ME: OK, how about 10 in the morning?
Person: Right. So what kind of [Model] [Car] is it?

ME: Um...it's a [Trim level]?

[Again, how on earth am I supposed to know exactly what bizarre sublevel of information is being requested here, or why it is relevant?]

In other words, why in this entire interaction was I repeatedly forced to guess in the dark about what are the reasonable answers to the question? They give the impression of utter flexibility, but wouldn't this be so much easier if they said up front that "we have slots available from friday" or "bring it in in the morning because it will take a few hours", and so on...