Addressing people in the ways they want to be addressed is one of those things that should be simple and easy, and yet people behave as if it is the greatest hardship. Let's talk about it. This is Allyship Sunday.
But a more important reason is about defaults. "He/Him" and "She/Her" have been the defaults for a long time now. And in many circumstances, it's only been "He/Him." But defaults are bullshit. Default create the false narrative that there is one "normal" way to be, and other abnormal ways. It's the same as the awful use of "skin tone" to denote a specific color, one that is certainly not the tone of skin of all - or even most - people.
But when being the default is easy, that makes being the exception difficult. If only people with pronouns such as "they" or "zie" have to state their pronouns, that makes them the exception. And why should they be? Because that's easy for the "default."
I'm not going to get into the long, long history of using Singular They in the English language. You can look it up. It's been around long before non-binary people took it up as one of the ways they wish to be addressed. Other terms are newer, but just as valid.
It's easy to be the default. It's hard to give up having something be easy. But that's one of the things allies must do. We make a thing a little less easy for ourselves, to make it much easier for people whose identities are marginalized by society, to help pave the way for those identities to not be marginalized. It genuinely is that simple.