Because the rules of the English language are the same for everybody. You don't just get to go around telling people they have to use a different ruleset for you or when they are around you. It's pretentious as fuck to expect people to cater their use of the language like that. That's exactly what names are for.

Certainly, the rules of the English language exist for a reason. For me, that's to communicate and convey information. If you unilaterally decide to change the language, then your words can fail to communicate what you intend. Although, I don't think that applies here. Correct me if you think otherwise.

If anything, it seems like the "new" pronoun rules you see today are people trying to enforce the standard English rules. He/she communicates someone's gender presentation. If you refer to "him" between someone who looks masculine versus feminine, then you'll assume that the "him" refers to the one that's masculine-presenting. That's how English usually works, and it works this way to allow us more efficient communication. Of course, this doesn't work 100% of the time because there are many axes and a gradient on each of these axes between what we consider masculine or feminine. What would the English language dictate when you're near one extreme on some axes and the other extreme on others? I don't believe we have any well defined and useful rules for this besides the preference of the person it's referring to. Again, do correct me if I get anything wrong.