Competitions are inherently unfair, especially physical ones. Some people are just naturally born with more athletic features and advantages. Also, considering the harassment, trauma, discrimination, etc. trans people receive, as well as the costs associated with transitioning, the chance that someone is transitioning specifically to have an advantage in competition is miniscule.
Trans men are men and trans women are women.
I don't doubt that athletes who transition are doing so sincerely. But, even if they're not doing it to gain an advantage, this does not mean they won't have one.
Athletic competitions are definitely unfair to the nonathletic. Sadly, I am never going to qualify for any major sporting event, even if I were to transition for a female.
In particular, about 50% of the population has a chromosome that typically grants a very large competitive edge. The advantage is so pronounced in most sports, that if this 50% and the other 50% competed together at the top levels of competition, nobody in the second group would ever be successful. As a society, we decided that segregating the two groups would allow more people to participate in athletics, and that's something we wanted to do. Allowing the group with the advantaged chromosomes to compete with those without it, unless that advantage could somehow be mitigated, would defeat the entire purpose.
Trans women are women. And cis women are women. The fact that they are both in the same category does not mean they are identical. If I were running a medical experiment to see how males and females reacted to a drug, it would make no sense to include women who have not transitioned in the female group. I'm all for treating trans people the way they wish to be treated whenever it makes sense. Sports are one of the few instances where it probably doesn't (although I'm open to data that shows that post-transition athletes do not have an advantage).