Why does Doffy or Kat not having access to that ability make no sense? Like, okay maybe you could say its odd that Katakuti doesn't when he has a mother who has all the skills needed to teach him if she wanted/he could observe her and attempt to learn on his own anyway.......but Doffy is basically self-taught at everything as far as we know (unless Trebol/Diamante/Pica were already Haki users when Doffy was a kid), and likely wouldn't have even seen any advanced forms of Haki in use until he was an adult.
And there are PLENTY of characters who despite theoretically being able to learn something just don't/can't. Some people just aren't talented enough in that aspect. Katamuri is apparently incredibly naturally talented in Observation and just somehow stumbled his way into learning Futuresight, despite it not being a linear upgrade that you automatically get from just having strong enough Haki. But him not doing the same for Armament or Conquerors shouldn't really be a shock. Rayleigh outright told us most people tend to focus on whichever form of Haki they're better at, rather than being incredible at everything.
Did any of that make any sense? I feel like I'm rambling.