First up kaido and oden
Imo 20 years ago when they were both coincidentally 39 the 2 were equal with them mastering their abilities to the best of their abilities just right before being able to hit that next level. That being kaido lacking awakening his fruit and oden his blades. As for why they never fully achieved those feats I think it 100% has to do with them just being too strong
Both were limited by their own strength in their own ways. Imo death is the key to both zoan awakening and forging a black blade. Something kaido probably figured out too late and maybe something oden just never learned before dying. It could also have been the case he never felt pushed to do so with him being comfortable in his own strength
We know both fruits and strong enough weapons are capable of having their own personalities and finding users they themselves feel worthy of them. I think it’s through death that they can truly become one with the user if they happen to be fully worthy to embody their will
Like using a fruit doesn’t mean you’re going to master it or having a meito doesn’t mean you’re going to make it black but hey that doesn’t mean that can’t find people capable of using them until they find someone more capable eventually
Imo you need to die in order to achieve zoan awakening but if you’re not truly worthy the fruit either takes over your body or it just moves on to find a new host.
I think black blades involve giving yourself up to the blade aka straight up killing yourself. If worthy you become one with the blade and if not damn…. I also think you can give up less if yourself for lesser gains. That imo tying to zoro’s eyes and shusui
If both lived longer both would have been able to achieve their true potential. Tho tbh kaido might return awakened after his fight with luffy
While not exactly the same because death isn’t a requirement for paramecia awakenings whitebeard and big mom might have also been in a similar situation. Tho tbh more so big mom
With how unnaturally strong they are without their fruits they likely weren’t put in too many situations where they really had to push beyond themselves and their limits
Tho with all of whitebeard’s scars and battles he’s been through throughout the years I do think he had to be awakened. That being what pushed him ahead of characters like big mom, shiki, and kaido and onto Roger’s level
As for big mom nobody really wanted smoke with her and her giant ass crew + territories. Even roger the pirate king himself rather be a bit sneaky and steal a copy of her poneglyph than go through an all out war against her. Probably because it’d just be a pain in the ass lol. Remember even shiki and his fleet were a big deal so I can only imagine trying to invade tottoland being even more annoying
Unironically she might have been deadass to strong and prepared to the point ain’t nobody wanted to deal with fighting her. With her essentially stun locking herself for decades unable to really grow stronger herself. Instead just lowkey getting fat, old, and kinda lazy since she spent so many years just waiting to do shit
Even when she was given opportunities to basically fuck up the balance whether it was getting the giants on her side or germa tech she got fucked over lol
Point is her being too strong and prepared essentially isolating herself accidentally stunted her growth. So instead of getting personally stronger achieving awakening she just got fat and old waiting because their was no real incentive