Why Didn't People Think That Voldemort Died After His Attack on Baby Harry?

After the Avada Kedavra curse rebounded from Harry onto Voldemort, Voldemort disappeared. I'm curious as to why so many key players believed Voldemort was still "alive" somewhere, his powers decimated, but, under the right circumstances, able to return to full power. It seems the witches and wizards in chapter one of Philosopher's Stone, The Boy Who Lived, were happy to believe that Voldemort was gone forever. Why didn't everyone believe Voldemort had died, rather than just disappeared? Was it the lack of a body? Or were there other reasons?

Here's a couple of quotes from Philosopher's Stone if anyone wants to review them. Otherwise, just disregard.

‘A fine thing it would be if, on the very day You- Know-Who seems to have disappeared at last, the Muggles found out about us all. I suppose he really has gone, Dumbledore?’

‘It certainly seems so,’ said Dumbledore. ‘We have much to be thankful for.’

_{Philosopher's Stone - page 13 - Bloomsbury - chapter 1, The Boy Who Lived}

‘But what happened to Vol– sorry – I mean, You-Know-Who?’

‘Good question, Harry. Disappeared. Vanished. Same night he tried ter kill you. Makes yeh even more famous. That’s the biggest myst’ry, see ... he was gettin’ more an’ more powerful – why’d he go?

‘Some say he died. Codswallop, in my opinion. Dunno if he had enough human left in him to die. Some say he’s still out there, bidin’ his time, like, but I don’ believe it. People who was on his side came back ter ours. Some of ’em came outta kinda trances. Don’ reckon they could’ve done if he was comin’ back.

‘Most of us reckon he’s still out there somewhere but lost his powers. Too weak to carry on. ’Cause somethin’ about you finished him, Harry. There was somethin’ goin’ on that night he hadn’t counted on – I dunno what it was, no one does – but somethin’ about you stumped him, all right.’

_{Philosopher's Stone - pages 46-47 - Bloomsbury - chapter 4, The Keeper of the Keys}

Answer

I think the quote from Hagrid that you used most eloquently shows that people had many differing opinions on the subject, that most thought that he had just disappeared, but some thought that he had died.

‘Some say he died. Codswallop, in my opinion. Dunno if he had enough human left in him to die. Some say he’s still out there, bidin’ his time, like, but I don’ believe it. ... ‘Most of us reckon he’s still out there somewhere but lost his powers.

As Keith pointed out in his comment, the lack of a body is going to give people food for thought. Additionally this is a world filled with magic, so even if there were a body there would likely be some skepticism as to his death.

It reminds me of the telephone game, where you tell a story (speak a phrase) to one person and they repeat it to the next and so on. By the time the story (phrase) gets around to everyone, it has fundamentally changed. No one knew what happened to Voldemort, not even Dumbledore (at this point), so of course there was speculation and many differing opinions on the matter.