It is not too different (attempting extra clarity) from people who would say "Oh but many think that AI is intelligent/not intelligent, <sneer>", but have little proper idea of matmul, of cognitive processes etc. (Imperfect simile, but may give an idea.)
If you accept you are just meat. Just mundane matter shaped in a way where it has thoughts. Then you accept your own true non-existence is inevitable. With no get-outs like returning to god or some spirtual unity with the universe or reincarnation or whatever.
It’s because of identity. Because people who are religious spent years and years and most of their lives not only studying and believing what they believe but also building community and centering their behavior around it. Abandoning that is the harder thing to give up.
If it were existential dread then we wouldn’t have entire countries like China being mostly atheist.
The DSM even has to include an explicit exception to prevent the clinical definition of “delusion” from applying to religious belief. Without that ad hoc exception, religious belief would be classified as clinically delusional.