Optimally queueing SCVs and marines and supply depots requires an APM of 11 or so in the early stages of a Brood War game on the fastest setting. Add a couple more APM for scouting, and we'ree still not talking crazy levels of multitasking.
Dealing with your opponent is a fact of every strategy game!
And yet if you watch low level players they’ll be fine with that until a bunch of zerglings show up at their base and then they panic trying to micro marines and repair bunkers while their minerals shoot up to 1000 and then they have no units and lose.
Keeping a scouting SCV alive in your opponent’s base while building more SCVs at home, building more barracks, building supply depots, killing the enemy scouting worker, and actually reading and correctly interpreting what your opponent is doing is non-trivial.
Dealing with your opponent is a fact of every strategy game!