"Why do mirrors reverse left and right but not up and down?"

They don't do either. They only reverse front and back.
Think about it. Look straight into a mirror placed perpendicular to the ground. If you are facing north (for instance) your reflection is a person facing south. It's because left and right aren't reversed that lettering and such looks backwards.

I grew up in earthquake-prone Chile, so from childhood I've been deathly afraid of earthquakes. Even the smallest tremor used to scare the crap out of me. Now I live in the US, and all we ever get is crappy thunderstorms and nothing else.
I've been through at least three quakes larger than 6, as far as I recall...