Because everything in the game is tech-locked and cultures are only allowed so many techs at the start of the game so not everyone is going to have it.* Cultures in 867 actually have a fairly wide variety of starting techs but this leaves certain cultures not having things like the ability to revoke titles** or able to recruit heavy infantry, something that has been around thousands of years before the start of the game. Its basically a flaw in the design of the game where there was a corresponding tech in each era that scaled up each age like increasing your MAAs, domain limit, and types of inheritance. AUH sounds like it might come with a major rework of how innovations works but who knows.
*I wouldn't be surprised if certain obvious tech that should be available aren't is a intentional choices of the devs to make things harder in a kind of arbitrary sense.
**If I remember correctly, French and Chinese culture at one point didn't start out with the ability to increase crown authority to revoke titles.