Because the glass formulation on iPhone 8 is totally different than is on iPhone 12 or 13. While its true those phones may have been harder to scratch, they were infinitely easier to break. Daily this forum was flooded with users all up in arms because a little drop caused the screen to shatter and they were very upset at the cost to replace said broken screen.
So Apple worked with (we believe Corning) to create a glass which is harder to shatter and that is the glass which first appeared on iPhone 12 and now on iPhone 13. And it is indeed harder to shatter (thought not impossible) as evidenced by the very few reports from users their screen cracked. But here’s where science gets in the way. It is not yet possible to create glass which is harder to break and EQUALLY harder to scratch. The glass in essence has to be softer (not a scientific explanation) to be able to withstand drops without breaking. But that means it can scratch easier.
I do NOT work for Apple. I am a user just like you are. I’ve been on this forum helping people for more than a decade. It’s kind of a hobby to offer help to people here.
If the above doesn’t help you understand, then there is really nothing more anyone can offer you. And again if you don’t mind scratches, don’t put a screen protector on your phone. But even an inexpensive one from a company like Spigen (you can buy a two pack with any installation tray on Amazon) for $10, is infinitely cheaper than replacing the screen on your phone.
Choice is yours of course.