I don't think it's that straightforward now. It's increasingly common for new cars not to be delivered with a full size spare, or a spacesaver spare wheel. Cars may be equipped with run-flat tyres. Not all tyre failures can be repaired with a can of puncture repair gunk. Added to that the hire company will then have to replace a repaired tyre, or refit a normal wheel when a spacesaver has been used before it can hire it out to the next customer.
It's probably an easier proposition for them, given the probability of a tyre issue, to do a proper replacement as soon as it happens rather than shouldering the increased risk of customers continuing with sticking plaster remedies.