Because the Tiger and the Dragon have nothing to do with each other in regards to Spirit animals. They are connected in Chinese Mythology. Trying to use the fact that Zoro's spirit animal is a Tiger to force him into the narrative against fighting Kaido, does not work.
The Mythological Symbolism of the Tiger vs the Dragon, The Yin and the Yang, the polar opposites yet similar. The King of Earth vs the King of Heavens/Sky. The emotional vs the level headed. There is nothing that connects a confrontation between Zoro and Kaido in this manner.
I didn't mention anything about spirit animals in my original post, I simply listed the traits of the Tiger in the Chinese Mythos and why somebody is ever called a "Tiger", it's due to those specific traits I listed. Then I stated that dependent on which website you look on the traits can be slightly different or very different between the Dragon & Tiger.
You're not really basing this off of any mythological chinese source, but just from the tvtropes website lists the traits that describe the Dragon as "laid-back" (like I said dependent on which website you look at the traits can be different to certain extents). But I now understand your source, so we'll stick with the tvtropes' description of the Tiger & Dragon for the discussion. The Dragon in our discussion of Tiger vs Dragon is already set, it's Kaido. We're now discussing whether the Tiger to that Dragon is Luffy or Zoro. But before we even get to that let's take a look at our Dragon here:
- "
Representing the dragon will usually be wise, tempered, patient, humble, plucky, and/or any other traits usually seen as 'laid-back'. "
Is Kaido wise? Eh. Is Kaido humble? ha.. Is Kaido patient? hmm..nope Tempered? idk about that..
So just from the start the tvtropes Tiger Vs Dragon doesn't apply here. As Kaido vs Zoro or Luffy is not a symbolic fight of "level headed vs hot headed".
This is actually my first time hearing about this "level headed vs emotional" stuff regarding the symbolism of Dragon vs Tiger in Chinese Mythology, and I've spent quite amount of time with this stuff due to Dragon/Sakazuki/Fujitora/Celestial Dragon related purposes. The Tiger & Dragon aren't really polar opposites in who they are, they're polar opposites in the "energy" they represent which is related to Dragon being the symbol of the ruler while Tiger being the symbol of the people who balance out the rulers.
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Luffy is not more hot-headed than Zoro, he's more adventurous/curious than Zoro:
-> Zoro was about to cut up the Celestial Dragon for pointing his gun at him.
-> Zoro wildly ran around Wano, Ennis Lobby, now at Onigashima
-> Zoro compared himself to a wild beast
-> Look back on Zoro's flashback, that's about as emotional of a dude you can get.
-> Zoro was wanting to go at Fujitora
-> Called a beast by Apoo
-> Man openly looks down on Sanji and anybody else he deems inferior. And doesn't even admit inferiority to Luffy. The one time he was humble is when he had to ask Dracule to train him, that's about it.
-> Stated to be not the type of guy to follow orders, aka wild
Luffy has plenty of level-headed moments as well:
- Arlong park with Nami
- Dude listens to people and understands their situation, doing so multiple times in the series
- The man told Zoro not to fight, when Zoro was about to slash up Bellamy and his goons. Not gonna get more level-headed than that.
When Zoro told Luffy to be careful, Luffy wasn't even being hot-headed, but simply not careful enough. But not even that Zoro himself was yelling this, while Luffy was normal about it. Who looks more level-headed in this panel, the guy yelling or with a mad face or the guy smiling and being chill in response to it and being humble with even apologizing?