You don't have to be a top prospect. You have to show it on tape in the NFL.
Kamara is lightning in a bottle. No RB in the league has his kind of acceleration.
David Johnson still might be the best back in the league (but I think Saquon has that title now) and no back is as good of a receiver as he is. I tout them as elite because besides being really damn good, they have qualities that no other back in the NFL possesses.
If your view of Chubb is that he's a big back with power who can get up to speed and take it to the house, I feel you're selling him way short. Yes, I'm arguing that Chubb is better than your description of him. That description sounds like Derrick Henry. I think big power backs are going the way of the dodo, and top end speed is pretty irrelevant if you need a freaking runway to get there. I think a much better comp for Chubb's potential would be like Le'Veon Bell before he slimmed down, or someone like Melvin Gordon. DeMarco Murray is another good comp.
Chubb having speed to outrun defense isn't an elite trait--at least by my definition, because plenty of other RB's possess that trait. When I say Chubb isn't elite, I'm not saying he isn't good--I'm saying he doesn't reach my top tier of 5 or so players that truly separate from the pack because they have talents no other back in the NFL has.
If you think Alvin Kamara and Kareem Hunt are comparable, I don't think you've watched them play.