Can someone please explain why the PS4/PS5 cannot play physical PS1/PS2 games?

I can understand why the PS5 cannot play PS3 games, but it has emulators for PS1 and PS2 which is how it’s able to natively download and play those games. The PS5 can play DVDs, which is most of the PS2 games were on, and Blu-Ray players can read CDs, so Sony would need to just give us a software update to do so. So the PS5 can already read DVDs and emulate PS2 games, so why can’t it do it off a disc??