OEM tires are made and priced to suit the auto manufacturers. In most cases their prime consideration is nice ride and low noise. Yes, they get a really good price due mostly to volume. Distribution costs are minimal when you are shipping to car makers. If you look at it from a roundness, balance, comfort standpoint, the tires going to OEM are at the tightest end of the spec.
The aftermarket, except for brand loyalty, is quite different.
Used to be that aftermarket tires had a wider spec than OEM on ride quality parameters. You could see that difference, for instance, by how much weight it took to balance a OEM tire as opposed to many of the aftermarket tires, as one example. At least one company, years ago, went to a quality standard called "OE or Scrap". They did not let any tires on the market if they didn't meed OE specs. There was some considerable scrap at first, but it didn't take long to meed that standard. Sort of a sink or swim thing for the plants.