We use 18 too.My opinion is it doesn't matter what WotC puts in their book. Every DM can decide for themselves what their max will be if they want something different.
No reason for anyone to get into arguments about what they think is the "right" answer. WotC will do what WotC will do, and everyone else can just make other personal decisions.
It neatly contributes to “fixing” the perception of Dex as the Uber stat too, at least wrt AC: light armour max AC 16, medium 17, heavy 18. Pretty sweet.
Of course that’s not the only reason, just a side effect… and if Uber Dex isn’t a thing for your table then it doesn’t matter.