Because cutting a mountain in half is an indicator of how well you’d do against any opposition.
Except those are Luffy’s peers. They are rival captains, they’re Luffy’s rivals, not Zoro’s. Killer fighting Kaido alongside the Kid and the rest of the alliance is different, as there is a singular enemy as opposed to a wide swath of enemies in the middle of the war. Also, Kid is not Luffy.
Btw, remember when the only reason Zoro even fought Kuma to begin with was because Luffy was incapacitated, after having to carry the crew on his own and fight the strongest enemy on the island while everybody, Zoro included, stood and watched because it was Luffy’s role and not theirs to fight the strongest enemy? And the fact that you even mention Morgan, who Luffy would have rolled regardless is ridiculous and not even remotely the same, as again, Luffy would have destroyed him and the scene itself was to show off Zoro, who would be joining the crew moments later. I treat Zoro’s relationship with Luffy like that because it is like that. The only other person that’s as close to Luffy as Zoro is Nami, and the three of them have a different relationship to each other being the three most important and original members of the crew, but that does not overcome the dominant theme of the entire story, reinforced by Zoro himself on EL and then again as a crew on TB, that Luffy fights the person that only he can beat, and doesn’t have help from the crew in crunch time.
Because it was a way to hype and draw attention to Oden, one of the three men, the other two being Luffy and Kaido, that the entire arc is predicated on, if you couldn’t tell by the fact that we are now in the midst of a flashback detailing Oden’s life.
