Because when you compare to Fury who has ducked Usyk and chosen to fight easy opposition outside of Wilder (Schwarz, Wallin, Chisora, Ngannou). Joshua was active as World Champion, fought his mandatories, and unified the division. The heavyweight division would be in a much better place if Joshua were World Champion now.
I agree but the first response when he loses always seems to be 'well he could have fought someone else'. It's sport. Sometimes you lose to the better man or at least the better man on the night.
It's a big reason why many fans of previous fighters/eras like Joshua: it's easy to conceive of him losing to anyone decent who comes to win. He's shown that he'll quit when the going gets tough (Ruiz), would rather lose on points rather than risk losing by KO (Usyk) and would rather win a safe points decision rather than go for a KO to avenge a humiliating defeat (Ruiz) or make a much-needed statement (Franklin). Wilder by contrast has shown zero quit no matter how tough it's got and always looks for the KO. It's night and day.