The main reason PSVR2 doesn't have pancake lenses is that those lenses absorb a huge amount of light.
You can't have HDR OLED displays and pancake lenses and experience the full brightness and dynamic range because of this.
Those headsets which do pair OLED (Or MicroOLED) displays with pancake lenses tend to suffer from poorer brightness, glare and smaller sweet spots due to the compromises needed to get the lenses working with OLED. (BigScreen Beyond and Apple Vision Pro both have this.).
Also the OLED displays on PSVR2 have pentile subpixels which reduces effective resolution and can cause colour fringing.
That said, the HDR on PSVR2 is supposed to be amazing. Really incredible levels of brightness and contrast.