I mean, it's mostly pointless, but PS1 & 2 emulation in software is stupidly easy at this point. It seems almost foolish NOT to include it. PS3 I can see being more of a challenge though.
Most likely the PS5 disk reader can't even read CDs at all, like the one on the PS4, so the concept of PS1 backwards compatibility makes no sense to begin with.
If you want a console to be backwards-compatible with games from previous generations through software, there are two ways to approach the issue: install a generic emulator on the console and run games through that, or manage games one at a time to make them individually compatible either through an emulator or by completely recompiling them for the new system.
The first solution is the most obvious, but it still requires you to set up the emulator in such a way that as many games as possible will work correctly, integrate it to some extent with the OS (for example for memory cards), and you are still left with a huge question mark on how many games will actually work correctly. The best PS2 emulators for PC have been fine-tuned by fans for more than a decade, but even though they manage to make most things work well or fine there are long lists of games with known compatibility issues or flat out not working at all.
So unless you want to spend a
lot of time and money for this, you still end up with an emulator that kinda mostly works, but is still not even a guarantee whatever game you meant to play will actually run correctly, so it's a difficult feature to market and assign perceived value in the eyes of the user.
The second way is to do the inverse process, so rather than creating a single emulator and throwing at it every old game in existence as is hoping a decent number works, you start going through the games one by one and retroactively fix what is causing compatibility issues. This is what Microsoft does for Xbox and Xbox 360 games: Xbox One is NOT backwards-compatible strictly speaking, but rather a list of about 600 older games have been made forward-compatible by Microsoft. When you insert the disk of a supported Xbox or Xbox 360 game the console DOES NOT run the game from your disk, bur rather uses the disk as a form of physical DRM and downloads a DIFFERENT version of the game from the store, a version that has been tampered with by Microsoft to guarantee it works.
In other words: this is the exact process that also happens when you download from the PS Store a PS1, PS2 or PSVita game that is compatible with PS4. The only difference is that Microsoft accepts an old original disk as a proof of purchase, while Sony doesn't and forces you to buy the game digitally even if you own a physical copy. I am expecting PS5 to be "backwards compatible" with PS1, PS2 and PS3 in a similar vein to how PS4 was and Xbox One and Xbox Series X are: so not "backwards compatible" at all, but rather supporting a list of select games they made forward-compatible. It's still unclear to what extent this is also the way PS5 is compatible with PS4 games, but going from Cerny's words I assume Sony could adopt a solution a bit in-between the other two (test the game, if it runs fine via simple emulation that's it, otherwise create a patch to make it forward-compatible). From what Microsoft said it sounds like XSX is actually backwards compatible with Xbox One in the literal sense of the term, but I don't know for sure.
So, at the end of the day you have two options: offer a quick and cheap, but haphazard and generally inferior solution to what Microsoft is doing; or go on and do what both they and Microsoft have been doing so far offering a select list of available titles (even though Sony decided to recoup the development and re-licensing costs from users who owned a game physically and Microsoft didn't, so in the general perception "Xbox is backwards-compatible and PS4 isn't").