It actually can't without being a reaaaally expensive console!

I can't remember the exact terminologies and such so you'll have to deal with the explanation I do remember.

The Xbox continually use the same type of chip for disc reading and the games are very similar in format. Which means it's just a matter of essentially unlocking the games to be played. Much like the Wii and GameCube games.

The PlayStation completely changes this in every single new console (apart from PS1 and PS2). Which means they would have to fit all the essential chips into the console to make it backwards compatible - which they did on the PS3 and it was a much more expensive model - or emulate, which they're kind of doing with PS Now.

Hope that makes sense and explains it a little bit:) I used to work in a tech store and got really curious about it one day so I researched, and am now passing the knowledge on.