Almost all cases, the phone landed on the ground (direct onto the face), once the screen protector cracked, and the whole screen including the actual display failed when the phone was dropped.
Why? Let us see, how the screen protector protect the phone.
A glass protector will spread the impact force onto a much larger area. This act can protect the top glass of the screen protect itself. And both the top glass on your screen and the screen protector have a thin layer of "glue", which acts like a shock absorber. So unless an impact goes deep enough so the pressure on the actual screen is too much only the protector should fail, once the protector failed, the screen would crack directly.
The other cases, the phone landed on the ground (direct onto the edge of screen protector). In these case, the glass protector failed easily under a little gentle shock.
So when we saw a glass protector cracked under a gentle drop without screen shattering, which did not mean glass protector protected our phone well. It mean the truth that glass protector is fragile, it is too easy to crack.
Why is it so easy to crack when shocked on the edge of a glass protector?
For it can’t spread the impact force onto a much larger area. So it just crack on the edge. Then the whole protector will crack after the edge cracking.
That’s the reason for glass screen protectors need to be replaced every time you drop the phone.