
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.
If you introduce yourself as they/them then its entirely reasonable to expect them to use those but if its any pronouns you are thereby giving options to people on what to call you, and most people in these instances will use the pronouns that are most commonly associated with the biological sex you were assigned at birth, because it is easier to remember rather than using multiple pronouns at equal rates, i doubt most people do this maliciously its just most convenient for them
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”.