Because ticket offices can't decide who they sell 'tickets in advance' to!
Imagine the scene, there is a short queue at Derby ticket office in the evening......
First Customer in Queue: Can I have an Off-Peak Return to London for the 13.01 on Monday?
Clerk: Reservations?
First Customer in Queue: No thanks.
Clerk: £61.50 please.
Second Customer in Queue: Can I have a single to London for the 13.01 on Monday please?
Clerk: Do you want the Off-Peak Single or an Advance ticket?
Second Customer in Queue: I don't know, it's £19.00 online.
Clerk: That's an Advance ticket, I suggest you buy it online and collect the ticket from the ticket machine over there [points to TVM] on Monday before you travel.
Second Customer in Queue: You won't sell me it?
Clerk: I'm not allowed to.
Second Customer in Queue: But you sold the last guy a ticket for Monday?
Clerk: Yes, but he only wanted an Off-Peak ticket, you want an Advance.
Second Customer in Queue: What's the difference?
Clerk: I only have to change the date on the ticket for him, but I have to book reservations for you and that takes longer.
Second Customer in Queue: But you could have just done all that by now.
Clerk: Indeed, but those are the rules.
I'm sure you get the idea.