The reason why WB and Mihawk have their tittles are they known as the strongest lol
From Mihawk's Vivre card:
大海賊時代以前より頭角を現し、今や名実共に「世界一」の座に君臨し続ける、稀代の大剣豪。
He has been around since before the great age of pirates and is now the world's greatest swordsman
in both name and reality.
Key point: "in both name and reality".
Kaido's entire thing so far seems to be only the perception of him, hence why Oda keeps putting his "title" in question.
Ch. 795:
"
People say, "in a one-on-one fight, always bet on Kaido."
"Land, sea, and air--out of every living thing in the world...
This pirate is said to be the strongest creature alive!!!"
This is the perception of Kaido, we don't know if it's the reality.
It would be simple to just say "This pirate is the strongest creature alive" but it doesn't say so.