Because things that require no thought or choice, like sending new workers to mine, should be automated, or given the choice to be automated. That's why rally-mine/auto-mine now exists - to let the player actually do what they want to do with a single decision.
Macro is not meant to make micro harder. Macro is the culmination of micro focused on ones economy. Selecting a worker and telling it to mine is micro. The effect it has of increasing your resource count to build more workers, or anything, is macro.
What you perceive as "pro macro skill" is just players getting good at fighting SC1's inferior UI. The UI of a game should let the player accomplish what they want to do in the most effective manner. Not having things like auto-mine and MBS and smart-cast in SC1 were actually UI flaws. It just so happened that people who could overcome those flaws better, played the game better, and started getting paid for it.
Is being able to fight a bad UI and do repetitive tasks really fast a skill? Yes.
Is it a good skill to make players learn that "enhances" gameplay? Personally, I don't think so.