Underlying everything is the science that says that a person who goes through puberty as a male gets permanent increases in bone and muscle density over people who go through puberty as a female.
In day to day life this ends up washing out in a sea of averages and individuals. In competitive sports, on the other hand, where we're no longer talking about averages but about peaks of performance, peak performers that experienced male puberty will always outperform peak performers that experienced female puberty. The advantage does not go away with hormone therapies, either. You'd get the same controversy if you let heavyweight boxers identify as welterweights too.
Many sports have handicapping systems that restrict or normalize natural advantages to allow fair competition. In track and field, for example, the most basic of these was the male-female split.
Lia Thomas is cheating the handicapping system. It's not fair.
But what I don't get is why women's brackets are so much more important than, say, racial or height or any other brackets.
As a cluster, are "people born with vaginas" more similar in athletic ability than "people born of a certain ethnicity" or "people who reliably ate at last 1500 calories a day" or whatever.
Presumably this goes the other way too, with cis women being better at ultra endurance sports than cis men. So what?
Like isn't the whole point of brackets to give lesser athletes a moment to shine in the spotlight, even though they aren't technically the best? So why don't they just make more brackets not based on genitalia?