Cars are specified by marketing departments and the accountants. If it's going to cost £15,000 at ex factory prices to put a car on 16-inch wheels and tyres, but £14,750 to put it on 18-inch wheels and tyres for which they can charge more, then that's what they're going to do. The tyremakers discount the low profile tyres because they know they will much more than make up the discounts in the increased frequency of replacement and increased cost of replacement of the 18-inch tyres.