Many people in this world don't believe there will be some postmortem surprise party. It isn't something that people like to think about too much, and I get it. Just as I get the appeal of throwing your hands in the air and letting Jesus take the cerebral wheel. Life is frustrating and complicated. And I'll admit that there are definitely more self destructive ways to drown out life's nagging questions by believing it will be all perfect in some do-over life to come.
Even the religious person who professes they'll meet their lost loved ones in heaven still mourn. The answer from outside a religious perspective is that life can be brutal sometimes. I won't claim that evidence and science provide a substitute for the emotional comfort some find in religion. Understanding we are fleeting inhabitants of this vast universe, part of something much bigger might have a beauty to it, but, it isn't the greatest comfort when processing your own mortality.
However, knowing that we only have a short time here can be a great motivator. It makes sure we make good use of what limited time we have. It forms a foundation for the curiosity that defines us as human beings, that drives us to contemplate our existence, educate ourselves, and to grow.