Because us bisexuals/queers/etc... were abused, forced into conversion therapy, assaulted, abandoned, and put into so many awful situations that you wouldn't survive one second of.
I don't ship Glinda and Elphaba nor do I see them as canon out of specific reasons since I first joined this fandom back when I was nineteen. I have no problem with that because at the end of the day shipping is not activism and not shipping them doesn't make me less attracted to women. However I am still a bisexual woman first and formost. The reason why LGBTQ is celebrated for a whole month is because it has historically been a battle to gain the same rights as straight people. For fucks sake homosexuality was considered a mental illness and could land you in jail.
And these things are not long ago either. Think about the HIV AIDS crisis in the eighties that got no acknowledgement from Reagan because as far as he was concerned they were "taking care" of the problem themselves.
So when you say things like "Why would Fiyero be excited with the fact his girlfriend/is Bi (if she’s not with Glinda and it’s not a throuple), and why would Elphaba be celebrating his sexual history?" you are actively doing what homophobes and conservatives do which is erase and belittle our history that we fought to keep. Sure you're talking about a ficitional ship with ficitional characters but YOU'RE STILL TALKING ABOUT LGBTQ. Please have sensitivity and nuance when discussing these topics.
I know it's hard to grasp because pride has become so capitalized, but it's origins are very much rooted in resistance and the right to exist. TO THIS VERY DAY, people are fighting to keep their rights and gay marriage is most likely going to be overturned here in the US. You're also more likely to be a victim of abuse, drug use, abandoment, and/or suicide if you're queer especially if you live in a red state and no, not because being gay/trans is mentally ill. It's because of people like you that support anti-human rights.
This has nothing to do with whether you like a ship or not. You're immature about it yes, but you need to do deep soul searching on why you are violently against the concept of celebrating bisexuality and queerness. Your family should be ashamed of you but for their sake I hope they never hear or see what you say online because it's disgusting.
But I don't expect you to understand considering you're a transphobic, Islamphobic, pro MAGA, pro ICE, pick me misogynist, z!o scumbag. Baffles me how you're a Wicked fan like did you fall asleep during the movie and show?
And no, liking gay ships doesn't mean shit but you are a piece of shit. I can say that about you because I have straight friends <3