Because if you break the display, your only course for a repair will be Apple or an Apple-authorized repair center.
Why is that?
It seems that Apple has tied the display -- yes, the display -- to the Face ID mechanism. This means that if you get a new display fitted, and the person fitting that display cannot carry out the proper pairing wizardry, then Face ID is dead.
Don't believe me? Here's iPhone Repair Guru with a couple of videos demonstrating the problem.
Now, not only are these videos impressive because we get to see someone so comfortably swapping parts inside a new iPhone, but we also get definitive proof that Apple has bound the display to the security system that deals with Face ID.
What this means is that if the display is swapped, the iPhone detects the change and disables Face ID.
So, if you break your display, your only current option is to go to Apple or an Apple Authorized Service Provider, and outside of AppleCare+, your out of warranty costs will be as follows:
iPhone 13 Pro Max: $329
iPhone 13 Pro: $279
iPhone 13: $279
iPhone 13 Mini: $229
Now, we've seen similar stuff from Apple in the past, and Apple released a iOS update to patch/fix/undo this issue.
It remains to be seen whether we'll see a similar reversal from Apple this time.