Maybe because gamers haven't moved an inch since the Xbox one failure and doing what they've done with the Pro is going to cost them at least two onion layers of their PS6 sales at launch if they repeat this situation.
At present I'm pretty sure the PS5 Pro is no where close to the aligned PS4 Pro sales in the UK, and with such a high launch price to disguise a massively bloated PS5 slim + Drive price they've boxed themselves in with the PS6 leaks/launch eating up their sales runway and killing PS5 pro hype IMO.
If they righted the wrong by doing a built-in disc drive, now they've still got a lot of other SKUs at the wrong prices for them to hit some belated momentum with a £549 price to offset the lost FOMO, and that change of price would also be a big issue for the early adopters that got taxed.
Personally I think they've completely botched the PS5 Pro release and potentially Xbox One'd the launch of the PS6 with the question mark of if the drive will be built-in for the PS6 OG.