"Because I say so" might mean "I'm too stressed to argue."
"Because I say so" might mean "I'll explain later."
"Because I say so" might mean "because I don't have time to explain it right now."
"Because I say so" might mean "I'm sorry you don't trust me implicitly, but please trust my judgment anyway."
"Because I say so" might mean "because I have to deal with the repercussions, I'm going to make the call."
But I doubt it. If it were those things, then why wouldn't you just say those things? I think we say "because I say so" when that's just how it ought to be but we can't figure out why. And each time it's a cop out.
If you can't figure out a convincing argument, then you're probably wrong. Admit it. Face up to it. Get over it. It happens to all of us.
There is nothing preventing you from explaining the reason for a particular need to a child. Certainly there are times that require quick action -- preventing a child from running into the street is the classic example. But you only act without explanation in the same way you would for a strange adult -- pull them out of harm's way and explain your behavior. Certainly, children will not always (particularly when very young) respond rationally to the disappointments of reality. But you can often still negotiate with them; and when they are very young, explanations don't make any sense anyway -- you do what you must.
"Because I say so" is lazy. It fails to take into account that the parent owes the child rather than the other way around.