One other thought I had was that the statements I'm describing in place of "because I said so!" are basically communicating with compassion, in a caring way.
It's not frustration or annoyance speaking, and it's not like barking something that (hopefully) will shut up the objections. That's a very defensive, antagonistic shut-down. (I think most of us dislike the idea of "Because I said so" for those reasons....we don't want to suggest that how children feel and what they think doesn't matter.)
But trying to root our decisions in "what is right" engages us in a subtle struggle that invalidates the "other" side. (Or their expressing upset challenges OUR validity, which is why it is draining and irritating to us.)
Conflict is okay. Disagreement is okay. Wanting a different outcome or answer is okay. When we own that we are exercising our parental power and making a decision in a situation, we leave room for dissent (we aren't trying to "answer" everything so that conflict & disagreement go away.) This was the aspect of speaking personally and owning my choice that changed things so much for me.