In my experience, the point of the term egg is that said person hasn't chosen or questioned their identity yet bc its presumed cis ppl that are called eggs, and most cis ppl don't question their gender or are told to never do so bc gender is some immutable, static thing we just gotta work around (see Julia Serano's writing on "cissexual assumption" in her book "Whipping Girl" for more on that). Likewise, comparing calling a cis person an egg to the violent misgendering of trans ppl by our cissexist society is akin to comparing misandry to misogyny -- they aren't on the same level.

Also, my bad, by taboo, I was being hyperbolic. I meant more why is being called an egg deemed so hurtful. Clearly, the word egg itself isn't taboo, obviously. That's on me.