Many don't actually know the importance of people around them until they are gone

That is what mostly happens. I think the reason for that is lack of observation and yes appreciation as you have mentioned. Lack of observation may lead us to come up with conclusions that shouldn't be drawn.

I also agree with @surrealfia, indeed life isn't long and isn't a forever long ride. So, the many memories we create if we take a closer look at it, maybe we will create different conclusions. Sometimes, small actions matter the most.