carrier I think one of the problems here is that, while transwomen are allowed to competre with females with a testosterone level of below 10 nmol/L (which is at the lower edge of what is actually normal for males - 9-38 nmol/L), the average range for females is 0.52–2.4 nmol/L. That’s a significant difference.
Also, as others have said, men and women of the same height have very different bodies - as just one example men have a lung capacity 10-15% greater than women of the same height - that’s 10-15% more oxygen going into the blood, allowing them to go harder and longer.
Of course, some of sport is inherently “unfair.” Not everyone gets to win. Most of the population is not fast enough, not strong enough to even dream of competing. However, even within the elite, we draw distinctions. Boxing, and martial arts have weight classes, because it is recognised that you can’t just have the heaviest people squashing everyone else (And this also recognises the different way in which the matches pan out in the different weight classes). This is also why men don’t compete against women. So, to allow someone with a testosterone level maybe more than five times that of an average female, and a male skeleton, and male socialization, training and development, to compete against other female athletes, on their say-so that they are a woman, is understandably causing concern.
As an example not to do with trans - do you remember when Oscar Pistorius wanted to compete in the Olympics - there was a proper and serious investigation into whether or not his blades gave him an unfair advantage against runners with feet. In that case “but he really wants to, surely room can be made for everyone?” was not something that was taken into account. The question is, does this athlete have an unfair advantage over the athletes against whom they will be competing.