Seems to me that even supposedly deontologic arguments usually have some (not always explicit) explanation,

Yes, and in my experience consequentialists usually have deontological sounding explanations for their choice of utility function.

Do you know real deontologists that really believe "Do X and don't do Y" without any explanation whatsoever? (How would they react if you ask them "why"?)

How would a conventionalist react if I asked why maximize [utility function X]?

Then all it needs is a charismatic priest who explains that, for some clever theological reasons, God actually does not mind you torturing this specific person in this specific situation.

And all that a consequentialist needs to start torturing people is a clever argument for why torturing this specific person in this specific situation maximizes utility.