Why is it easy to prove someone is dead but not so easy to prove that they're still alive?

I am not sure if I already asked this but help me understand the thought process here.

My dad, who retired last year, became a victim of this. He got a letter in the mail stating so and so was sorry for the loss of him. It took him a long time to get everything straightened out. I’m not sure if he’s still dealing with it or not but he was so stressed out by having to prove whoever sent him the letter that he was still alive.

Why is it so difficult to prove you’re alive? I think it would be easy but apparently not so.