Why do journals need styles?
There are several reasons:


Publishers want their papers distinguishable. Everyone1 knows Elsevier by the logo and font.
Consistency itself is a good goal, to an extent.
It means you easier meet certain quality standards. (If I could, I would show you things people submitted as final versions for publication.)
I'm afraid that if you let people do what they wish, it would be seriously disasterous. (I speak from a Copy Editor/Typesetter experience.)
I would not call it inertia, from my point of view, journals are typography and typography is art, and art should be done artfully, no matter we live in a crazy fast-cooked world.


Last but not least, note that AMS does not require you to comply with everything, I published a paper where some enumerations are numeral and some are alphabetic.



1Almost.