Phil and Shel pretty well summed it up. To give you an example, my youngest was given an 11-piece set when he got married two years ago. To date they have used 4 of those pieces. But they use those four over and over again. The others just languish, taking up closet space.
One additional thought: Very often you'll find that one manufacturer's make and model is ideal for your cooking style in, say, a skillet. But a different maker builds just the right saucepan for your needs. This ability to mix & match comes from buying out of open stock, rather than sets.
FWIW, I feel just as strongly about this when it comes to buying knives. Probably even more so, because with knives there should be no compromise when it comes to the fit, the comfort and the suitability for the purpose. And sets just don't make it in this regard.
They have taken the oath of the brother in blood, in leavened bread and salt. Rudyard Kipling