Most flats come from something sharp picked up off the road, and a good number of them can be repaired and put back into service for years.
A proper repair is not a plug pushed in from the outside at the roadside. We take the tire off the wheel and inspect it from the inside, because that is the only way to see whether the casing has been damaged by running on it flat. Where the puncture qualifies, we fit a combined patch-plug that seals the hole and the inner liner together.
There are punctures that cannot be repaired safely. We would rather lose the sale than send you out on a tire that might fail at speed.