Because how can you go to a bunch of seminary girls and say to them “Everything you do you should do in order to serve hashem”? That’s totally unrealistic! They have countless reasons to do everything they do, none of which has anything to do with the torah. Should they just “turn it off” and replace it with hashem? they can’t do that, (nor does the teacher)
I just don’t see how that’s a practical guideline by any means. They’re just pretty words.
Let me give you an example: Am I posting here to serve hashem? No I’m here because I enjoy it. So you might say: “yes but by spending a certain amount of time doing things that you enjoy you get the energy to spend time later doing mitzvot”. But that’s just another way of saying serving hashem is not so important to me. Because if it was, I wouldn’t need so much time reenergizing.