The slightly longer explanation
The overheads to do print-on-the-day are much, much higher. It needs a lot more people and a lot more equipment, all of which have to be paid for. This has to come from the money we make on the day, so we'd need to make more money, which in turn means we'd have to charge you higher prices. So instead of charging you £6.00 for a 7x5 print, we'd have to charge you £12. You'd literally have to pay twice as much and we'd be working twice as hard for the same result.
By not doing print-on-the-day, we can save loads of money which we can pass on to you, so you only pay half the price and we only work half as hard.
We sort of like that idea because everybody wins :-D