Unicode does not have different codepoints for every language. Leaving aside scripts that aren't encoded at all, all languages using the Latin script get a single set of glyphs, and if you want Comic Sans MS or Carolingian miniscule or fine-grained control over whether "a" has a hook at the top or not, you need to specify a font that renders it the way you want, just like with 直.

Because Unicode is about the meaning of characters, not their appearance. If there were a context where the two different shapes of 直 have different meaning, Unicode would add a new codepoint to distinguish them. In fact "ɑ" without hook does have a separate codepoint because it is used in linguistics to mark a vowel different from "a".