Why? Well to have a proprietary device which only Sony sells instead of using off-the-shelf SSD - at premium prices, of course.
??? Did blu Ray expensives , Did DVD expensives . Not all the things made by Sony are expensive . Also why would they one to fuck their product . Also their newest statement said they want all PS4 player to move to PS5 as fast as possible, which is lead an affordable console's on mind.
They are not stupid, unless you have a weird opinion with Sony or Playstation as brand.