It’s not that they’re not improving like they used to, it’s that the die can’t shrink any more.
Price cuts and “slim” models used to be possible due to die shrinks. A console might have released on 100nm, and then a process improvement comes out that means it can be made on 50nm, meaning 2x as many chips on a wafer and half the power usage and heat generation. This allowed smaller and cheaper revisions.
Now that the current ones are already on like 4nm, there’s just nowhere to shrink to.