Because Sony got greedy over a few cents per unit. The last Sony console that could play cd/ cd audio etc... was the PS3. I noticed this right away when I got a ps4 on launch in 2013. No Compact Disc logo on the box. The ps1, ps2, and ps3 had it. They have to license the tech, spend 20cents per ps3 or what not. Apparently the bean counters didn't want to do that. Thats a few grand in profits they loose.
You may ask what does cd's have to do with Dvds. Well the audio portion of a dvd needs the cd audio format, and they aren't going to give ps2 bc without also giving ps1 bc. PS3 had it.
Why Sony did this , I don't know. At the time they were pushing their digital music service, but they are not anymore so no clue why its not in the ps5. It sucks. It would also suck for me though personally, as I got rid of a lot of my ps2 games, outside of the rpgs. I got ps3 collections for silent hill 2 and 3, not knowing that the collection would be ps3 only and never again to be relaunched. Meanwhile my ps3 is on its last leg. I should of kept the ps2 games as they are now worth $80, and i still own a ps2. Ps2 is working fine, ps3 won't even connect to wifi or bluetooth.