Oda has never shown Roger, Garp, Sengoku, or Prime WB fighting at their absolute peaks all the way to the finish. The closest we ever got was Roger vs. WB for three days, and even that happened offscreen.
People don't understand that Kaido was the bogeyman of the entire old generation. It's a pretty simple sequence of events to follow:
- During the era when Roger and Prime WB were at their peaks, Kaido became the very embodiment of strength in the world.
- He conquered Wano and became virtually invincible.
- He tanked Oden's strongest attack and went on to defeat him. The same Oden who was on the level of Roger, Prime WB, and Xebec.
- He killed Oden, who was beloved by both Roger and WB, enslaved an entire country, and even Prime WB chose to avoid fighting him.
- He only grew stronger from there, and it would take another 20 years for him to finally fall to Joyboy, the man destined to surpass Roger and everyone else, after running an entire gauntlet while carrying an island through the sky all night long.
It's really that simple. Kaido was an obstacle that neither Roger, nor Whitebeard, nor ANYONE ELSE could overcome. Only Joyboy, pulling off his miracles in a 20-vs-1 gauntlet, could finally bring him down.
You can clearly see that Roger and Garp look less impressive than Kaido.