Because trans people want to participate in the category of their gender and not be discriminated against and put in an extra category? When all data so far points to trans people not dominating their gender category in sports why would you separate them and forcibly out them? What would the category even look like? Trans men separate, trans woman separate, enbies separate depending on hormone configuration? Well now you got your medical tests back. It's way easier to put them into the category of their gender where they can participate in a fair way. Most cases of the "evil trans women dominating x sport" is some kind of misinformation that even left leaning folks often fall for.

Example of Lia Thomas who was supposedly low end of men's category and then won in women's category. In reality she was very damn high in the men's category before hrt, then very low in the men's category on hrt (this is the point the misinformation classifies as her best in men's category), then won one match and placed average in a lot of others in the women's category (the one win is the point the misinformation makes out to be the norm instead of all the losses and average placements).

It would be much better overall to not group people by sex but instead group them by competitiveness. Like weight classes in boxing, sport competitions should use other measures to decide who competes against who.