Because if you made a universal law of bullying for all rational beings, then everyone would, by law, be required to "bully" every other rational being. In short, for Kant or other deontologists bullying would become a/n (absurd) "universal law". Absurd and/or irrational is plain WRONG for Immanuel Kant, etc. Quote:
So we have a law the thought of which can settle the will without reference to any expected result [i.e non-consequentialist and duty-prescribed; a.k.a. "deontological" ethics; KB], and must do so if the will is to be called absolutely good without qualification; what kind of law can this be?
Since I have robbed the will of any impulses that could come to it from obeying any law, nothing remains to serve as a ·guiding· principle of the will except conduct’s universally conforming to law as such. That is,
I OUGHT NEVER TO ACT IN SUCH A WAY THAT I COULDN'T ALSO WILL THAT THE MAXIM [personal rule/preference; KB] ON WHICH I ACT SHOULD BE A UNIVERSAL LAW.
In this context the ·guiding· principle of the will is conformity to law as such, not bringing in any particular law governing some class of actions; and it must serve as the will’s principle if ***DUTY*** is not to be a vain delusion and chimerical concept. Common sense in its practical judgments is in perfect agreement with this, and constantly has this principle in view. END-QUOTE [Groundwork/Foundation of The Metaphysics of Morals; 1st Section at 402]
Of course, Immanuel Kant couldn't possibly rob anyone's will of anything as he fatuously asserts above. But it does give anyone with ears to hear (German philosophers) the very good reason why so many German people said they were just DOING THEIR DUTY or OBEYING THE LAW when asked about their roles in abetting/allowing/participating in the Nazi led holocaust. "VE VUS CHUST VOLLING ORDERS as was required by our DUTY to the Reich UND FUHRER!" --- which makes perfect sense if you actually believe Kant's moral thinking (or lack thereof)