Because in the end you’ll be dead.
Everything will not be alright while you’re alive and you won’t reach a point when everything is as you imagined it would be.
That sounds pessimistic, but it’s actually just acknowledging the fact that though situations can improve and life can be good, it will never be without problems. Life is inherently problematic (which is why we’re natural problem solvers).
It’s important to acknowledge that, so we’re not always obsessed with thoughts about an ideal future, only to find out that the future is a lot like the present.
All we have are present moments. We should live them, instead of always thinking about what we don’t have.
I hope heaven exists, and I believe it does in some form. But we experience only moments of it in this realm.