My understanding is that the spark plugs are removed every 4 years in order to prevent them from seizing (in the olden days we used to clean and re-gap them every 3000 miles then replace them every 6000, but these days spark plugs are good for 40000 miles, and so they don't get removed regularly).
In theory, you could clean them and continue using them, but he plugs themselves are not extensive, and the logic is that if they are already out, you might as well just put new ones in.
Furthermore, modern cars will have all sorts of things that need to be disassembled for access to the spark plugs, and on V6 and V8 engines getting the inner plugs out is a bit of a nightmare, all these things mean labour cost, and in the UK parts may be cheap but labour is expensive - and so replacing the spark plugs while they are already out does make commercial sense, even if technically they could survive a few more years or miles.