Because science tells us through observable evidence that when our heart stops, your brain activity stops soon after, and brain activity and consciousness are observed to be related. And right or wrong, those observations are real things that are repeatable. Every time biological processes stop, consciousness stops. Every time. Not 95% of the time.
Instead of asking me to prove that something doesn’t exist (a favorite retort of religious people), why don’t you use science/reasoning/evidence to prove to me that it does exist?
But you can’t. Not even a little. Which goes back to the “because I said so” argument that you religious folks fall back on. You’re told from your higher ups just to tell people that it’s something called “faith,” which is nothing more than blindly believing a fairy tale that your parents and every generation before that passed down. That’s how you get around everything, by just saying you believe something that has no evidence or reality other than your mind being so brainwashed that you refuse to consider anything else. You’ve been told a story for so long and so often that you just believe it, and science has proven that if you hear and tell a lie long enough, you’ll start to believe it. Add to that the fact that you’ve spent your whole life putting a make believe figure at the center of your consciousness, and it’s easy to see how you’d have a giant panic attack inducing existential crisis if you realized you’ve wasted all that time and energy on bullshit.
But...it will pass eventually and you’ll see the world in a better light. Google Jon Steingard. He’s just one example of proof that you feel better when you’re not under the weight of bullshit you don’t want to admit you know is bullshit.