Because it's a physical thing. And Hopkins was never known for his stamina. To show tremendous stamina you must show either a ton of movement or a volume of power shots. Something he didn't do.
Even there I wouldn't even put him near the bottom, let alone "least of all time". Stanley freaking Ketchel? George Foreman? Carlos Monzón? Harder punchers, sure, but that's a very small part of the definitive framework of athleticism. I'd say agility, balance, reflexes, and kinetic proprioception are much larger and more weighty factors than power, and Hopkins beats all those guys hands down in those categories.
Monzon might be the strongest fighter I've ever seen p4p. Rarely will you see a guy just throw elite fighters around the ring like they're kids.
Yeah but would you call him more athletic than Hopkins? Sort of a klutz in comparison, IMO, so I wouldn't be able to agree with that statement.
It's also important to note that Hopkins essentially willed himself to greatness. He wasn't some young teen that a trained eye picked out of the bunch and sculpted to greatness. He was largely self taught. He lost his first fight. He's rather the opposite of a Sergio Martinez - a fighter who walked into a gym in his 20's and had the apparent natural ability to make it to the top. Instead, he's a guy with an exceptional mental aptitude for the sport who never stopped learning.
Very good chin? He's got an Iron Chin. But aside from that I agree he isn't very exploive, fast but he's just very allrounded.
Hopkins had the toughness and work capacity to hang around for as long as he did, a well as the reflexes to minimize damage. A lot of that is his brain, but he's definitely pretty athletic.
Frazier. If you have any doubt, just watch the 1973 Superstars. https://www.youtube.com/watch?v=WUTxgTEdZTA https://www.youtube.com/watch?v=qTKDDTI36yE
Hops is/was the embodiment of all the traits that make up a good/great fighter. He had decent power, enough to keep you honest. Had enough speed to keep you on your toes. Enough quickness to make you miss. Enough reflexes to catch you when you made defensive mistakes. Enough stamina to take you late into fights. In other words he was probably one of the most well rounded fighters to ever grace the ring! No one particular thing about him was world class but, the fact he did almost everything very well, not great, made him world class!