I believe you’re conflating attraction with how one chooses to date.
To a bisexual person, I assume, their attraction is comparable to a man who likes both fat and skinny women. In that instance would you ask the man, “since you like both fat and skinny women, why not have the option to have both at the same time?”
See how it doesn’t make sense logically? The root cause is sexual attraction, it does not dictate how one chooses to date or automatically make them polyamorous because they have more options in terms of what they are attracted to. Straight men and women are attracted to people who they aren’t dating - under your logic you would ask “well why limit themselves, wouldn’t a poly relationship work better since they’re constantly checking out random people of the opposite sex?”
Sure you could argue that “well a straight man would fukk any woman, as long as she’s a woman, so it doesn’t count.” But by that same logic is irrational because you could say that same man would have sex with a female to male transsexual because they are “technically a woman” even though they could look just like a man. So that is illogical thinking as well.