Your simple regex will take you 1-2 additional minutes to deal with prettified code like this:

  <button
    href=".."
    class="...."
  >Button</button>

And when you start fixing more complex components and HTML edge cases, you will have a lot of fun [0]. This is fine for legacy code. But I don't want to deal with this in modern projects, if I can just change a single file and be sure all components are updated. As you said, this is not related to functional CSS, but it solves many disvantages of it.

[0] https://stackoverflow.com/questions/1732348/regex-match-open...