When No Person shows Greater Power than WB, They have No Choice but to call WB "Strongest Man"
When No Person shows Greater Physique/Body than Kaidou, They have No Choice but to call Kaidou "Strongest Creature"
When No Person shows Greater Swordsmanship than Mihawk, They have No Choice but to call Mihawk "Strongest Swordsman"
When you're the Best, you don't need to prove yourself to anyone, you just Claim the Title & it's up for others to Prove that you're not
Simple Answer is: "They Failed"
There are Dozens of Characters who have DFs as Powerful as WB's, but He is only one whose Power threatens entire World
There are Dozens of Characters who are as Physically Strong as Kaidou, but He is only one who is Near-Unkillable
There are Dozens of Characters who have Top Sword-Styles like Mihawk's, but He is only one whose Style cannot be Countered
Surpassing WB required someone who did the Impossible by having Two DFs (Historically never Happened, for at least 800 Years)
Surpassing Kaidou required someone who did the Impossible by Awakening Nika DF (Historically never Happened, for at least 800 Years)
Surpassing Mihawk requires Zoro to do an Impossible Feat that was never witnessed in at least 800 Years
And coming to your question about Mihawk specifically, it's as i said above, They all Failed to prove that Mihawk doesn't deserve his Title, instead of saying "Mihawk didn't Defeat this & that & that", it's the opposite, Mihawk's Title signifies that the World didn't see anything from Shanks or Oden or Fujitora or any Knight or any Samurai or any Pirate that makes them Claim this Person deserves Title more than Mihawk
And Shanks is biggest Candidate who Hypes Mihawk, even if They never fought, cuz Shanks is Extremely Famous, if the World didn't acknowledge his Power, They wouldn't call him an Emperor, They wouldn't give him 4B Bounty, They wouldn't be scared of him, Kaidou wouldn't put him in his Top 5 He ever Faced ... etc
Despite all this, Shanks was simply not given such Title, and Brannew even confirmed it
Mihawk is simply Stronger than any Swordsman