Insufficient evidence to the contrary.
I think I saw you talking about reincarnation. Even in that scenario, I still think you might as well be dead forever. The things that makes a person who they are--their memories, their personality--these are shaped by your physical existence. If I have a soul that survives as a slate wiped clean, I think that's interesting but I don't see why I should care about that anymore than I should care about knowing that a donated organ will survive my death. I don't believe that thing that would survive is me.
I'm not one for idle speculation. I want facts. Some children purportedly return with fragments of memories from being someone else. There are stories galore, from the mouthes of the babes themselves. Some having knowledge at a very intimate level that defies being coached up by parents.
In addition, most people, do not remember a thing. There must be reasons for this.
Control, as far as you are able, is the reason one should care about all this. The world is riddled with potholes.
It makes life special, you get to exist for 80 or so year in average. I think about this a lot and I'm finally feeling "okay" about it, it's weird to think that we just stop existing but you will not suffer or even have a way to experience non-existence at least.
Enjoy it, one day you're just going to sleep and stop existing and that's okay.
The moment to believe in something is until there is sufficient proof. As there is no evidence of an afterlife, there is no reason to believe in an afterlife and I will continue to live my life as if there is none.
It doesn't mean that I deny with a 100% that there is no afterlife, but you could say the same about basically anything else.
That's the reason people "are so sure" of it.
if you want proof you'll have to seek it out