It's not just about strength. Even if Kaido is just as strong as Luffy or even weaker, he still has a much bigger crew. Did way more atrocious things, held more territory, has weapon factories, and he probably has more allies/subordinate crews. All of that is factored in determining your bounty.
it's already third time i am forced to do this but ok let's make a comparison
kaido has what you just said.
1) adv forms of coc haki. all of them
2)strongest ( or most feared ) df in verse awakened. not mastered yet but sure will soon
3) a prettey decent fleet.
4) 3/4 red glyphs and the only person in verse able to read them
5) 1 to 3 ancient weapons ( poseidon confirmed is one, zounisha might be the second if confirmed he is uranus and actually pluton too if fraky decides fuck it i build it
6) stronger crew by far at this point
7) massive influence over : revolutionary army,wano,zou,fishman island,amazon lily,alabasta.
8) 2 warords as allies one of them as a direct underling
9) feat way more massive than all yonkos combined ( destroyed eneis lobby,massacred the totality of cp9, successfully infiltrated AND escaped from impel down ALONG over 200 very dangerous pirates including fromer warlards he ALSO previusly defeted in a sinlge day. punched a celestial dragon.
courrent luffy makes the wg look like his little bitch.
if he was even remotely evil or interested in actually destroying the world he could ammass all of the above in a couple of weeks than go to mariejoy streets in front of the wg hq, do this with apoo literally making the background music for him while he does that, killing them all, and NO one could do shit about it
who is kaido again as of now? a big dude ( if even alive ) with a mythical zoan and a bunch of other big dudes with other zoans and a couple of destroyed factories
wow. yeah. he needs a bigger bounty than luffy alright...