Stephen Hawking has died at the age of 76.

[Anomalocaris]

Why would he believe otherwise?

I didn't ask for reasons to believe otherwise... I ask for reasons to believe the following statement

"Hawking is only in our memories now, he isn't in an afterlife just like he didn't exist 4 billion years ago."

For the exact same reason we believe there really isn’t another roadrunner79, identical to you in religiosity as well as all your other less disreputable salient foibles, presently roasting on the sunward side of the moon.  The reason is that nothing in what we really demonstrably know, as oppose to merely wish to believe, vaguely suggest that could even be infinitesimally probable.