I don’t say, “Because I said so,” pretty much ever, either as a teacher or as a parent.
But I do something similar:
Me: Please go pick your coat up off the floor.
Kid: Why?
Me, after an appropriate length of time: [bugs out eyes, scowls like a psychopathic emu] I thought I told you to pick your coat up! Why is your coat still on the floor?
Kid: But-
Me: No buts! If you want to discuss the issue, that’s fine, but your coat is on the floor! Pick it up, please, and then we’ll talk!
Kid: Aaugh! [Picks up coat]
Me: [Manner instantly softens] Thank you. Now, did you want to ask me a question?
Kid: Why did I have to pick it up?
Me: First, I really appreciate your picking it up. When it’s on the floor, there’s a few problems. It can get stepped on, and other people can trip on it.
Most kids: Oh, okay.
Some kids: I don’t mind if it gets stepped on, and other people should watch out.
Me: That’s fine, but you asked for an explanation, and now you have it. I expect not to see your coat on the floor again.
In other words, compliance comes first. After compliance comes an explanation, if the kid needs one and if there’s time (I’ve also told kids to ask me at recess or whatever if it’s currently busy). And sometimes their reasoning is good enough that I apologize for my initial direction and reverse it (“But that’s Dave’s coat, not mine!”), but that’s honestly pretty rare; most of the time their objections to following directions are pretty silly.
I used to be much more of a negotiator with kids, but my experience is that negotiation before compliance leads to a lot of wasted time and energy, and leads to more conflict, not less. If kids know that they can get an explanation when they need one, but that they must always comply first (with an exception made for situations in which compliance would be dangerous or cruel), they actually find themselves having a better life, inasmuch as I generally have a good idea of what they need to be doing.
With most kids, I build up that trust, and pretty quickly compliance-with-questions becomes second nature. That can’t happen if they can get away with arguing before compliance.