why shouldn't adults bully each other?

[Esquilax]

I see no reason for moral systems. can you cite why we need them?

Because we're social animals that require each other and our broader society in order to live the comfortable lives that many of us do. In an effort to maintain this society- in the past this would be a way to avoid getting fucked up by any number of environmental factors, but today is mainly about keeping up our level of comfort and convenience- we agree not to operate in ways that harm one another. It's a social contract we all opt into, in order to keep things running to our advantage.

Do you like the society that keeps you sufficiently alive and vested with wealth enough to spend your time on an atheist forum through a space age magic box that only a few decades ago would have been science fiction? Me too! So you see, we're both invested in replicating behaviors that allow that structure to keep ticking over.