
Because it’s still hurtful to give people ALL the options in the entire language and they return no effort to you. In another way, I assume you like multiple foods, right? If you said “hey I like ravioli and also lasagna” and people ONLY served you ravioli for every single meal, it would get exhausting. If you heard “well I think you don’t look like you enjoy lasagna, so I don’t believe you. So I’ll just give you ravioli” frequently, you’d get tired of it.
One last thing (I don’t wanna sound preachy, so sorry): I do know it’s possible because I have people in my life who do switch between my pronouns. They’re not any smarter than anyone else, they’ve just put in a tiny amount of effort more than others. And I know that it’s a tiny amount of effort because I do the same for other people who use multiple pronouns.
I also don’t believe it is malicious, however that does not mean it does not hurt my feelings. I make efforts to appear NOT as my birth sex, and so I think it’s reasonable for people to use pronouns I give them. (It’s reasonable to expect proper gendering regardless of appearance, but anyway.) I also list she/her last in my set of pronouns because they are LEAST preferred, and I usually say “they/he/she or just they/them”.