Because under the glass there is not just a board and flex cables. There is a year of life: a child's first steps, the last holiday with parents, the cat that slept so funny in a box.
The good news: a broken glass almost never means the photos are dead. iPhone memory is not photographic film exposed by an impact. The data sits on the storage separately from the display, and damage to the screen by itself does not erase it. It is another matter if after the fall the phone has started living its own life: rebooting by itself, not turning on, or asking for recovery through a computer. That is where it is important not to play the hero.
The first thing a technician does when someone brings in a broken iPhone with the silent question "are the photos alive?" is not to grab a screwdriver. First a conversation: what happened, when it stopped working, whether they tried connecting it to a computer, whether there is a backup in iCloud. Then diagnostics. The phone is connected to a power source and they see what it is generally capable of: whether it turns on, whether the system sees the storage, whether it responds to the computer's requests. Often there is already more data at this stage than the owner had: the person saw a black screen, while inside the phone was quietly booting and just waiting for a new display.
If the storage is intact, recovery is most often a matter of care, not a miracle. The data is taken off through a special mode, copied to a known-good device, and only then does the phone go in for repair. Display repair does not threaten the pictures, so there is no need to fear a screen replacement. What is more dangerous is not the replacement itself but panic: attempts to "just try once more", random self-initiated resets and firmware flashes following advice from comments. It is these experiments that turn "the photos are inside" into "the photos are gone".
So if the screen is broken and there is a life inside, take the phone to those who know how to separate one from the other. And do not delay: the sooner the device gets into a technician's hands, the higher the chance that your cat will stay in the box forever.