They had the interest in sports before the transition, which means that as men they were very good at it. Which explains why there's loads of trans women who place very highly in female leagues while there's almost no trans men in male leagues.
I'd place the emphasis a bit differently. Let's say there are 300 men in the NBA and 200 women in the WNBA. These men and women are at the far end of the distribution in basketball skill for their respective genders. If there are 3 billion each of men and women in the world these are the top .00001%. But if you took the worst woman in the WNBA and put her on the men's distribution she might be down to the top .01% or so. This means that while she is better than almost all men, there are still hundreds of thousand of men better than her.
You can observe this to some degree. If you go watch a decent high school basketball game around here there will be 5 or 6 players per team dunking in the pre-game layup line. You will see 10 or so in game dunks.
There have been a total of 5 women with 19 in-game dunks in WNBA history, despite the fact they use a smaller ball. They get about 1 per year.
https://www.wnba.com/history_triple-doubles-dunks-and-20-20-games/
The NBA is on pace to have about 11,000 dunks this year.
That's a special case--basketball is a sport where height has a considerable advantage and men are slightly taller than women--which means at the extremes there's a lot more men of a given height than women.