Because there's utility in a catchall term that is lost due to the complexity LGBTQIA+ imposes. Whereas on the other hand sexuality and romance are very complex so for individuals more specification is necessary to understand the totality.
This does pose a problem in casual conversations so I just use "Queer".
Basically this:
Okay do you understand the contextual difference of a trans person self-referentially adopting "Tranny" or a queer person irreverently adopting "Faggot" (
example) and you throwing it at us? While I'm okay with trans people calling me "General Tranny" or such, if you could refrain from hurling slurs at me like you did I'd appreciate it.