The phone alerts sound quite dystopian and unsettling. As if the earth shaking and feeling of imminent doom wasn’t chaotic enough, now you have yet another element of annoyance and distress added to the mix as a cherry on top. Just enough of a distraction in a life/death situation to tilt the scales in a negative direction.
Knowing this, if you get the alert, get to safety immediately.
That alone can help save people.
5 seconds is still time purchased for those who use it wisely.
It could help people react better too by confirming what they are feeling is an earthquake and not a train or plane or your neighbors. It could also wake someone seconds before so they are more aware when it happens and not awakening in primal panic mid quake.
It isn't a huge help, but broadcasting to everyone in range of certain towers shouldn't be dystopian.
They sound that way to be unique and attention-getting. They have to cut through the background noise of 21st century life.
Here's a (totally unsafe!) demo from the 80's of why sirens sound the way they do. Do not attempt at home.
https://www.youtube.com/watch?v=Lsfczwy3KOE