I'll agree with you that the number of males who transition for the purpose of competing in sports is vanishingly small. (I can't think of an example.)
Questions that begin with "why" rarely have answers as simple as people want.
Why does someone transition?
That's a really complex question, and it varies from one person to another. Why do some people choose a little bit of transition, while others choose a lot of transition? When it comes to something that big, it is probably impossible to distill it into something that can be expressed in a sentence or two. Moreover, the person making that kind of major life decision often can't express why they want to do it. They may not even be aware of it themselves. Psychologists study people and often see patterns of behavior and decision making that point to an obvious motivation that the object of study (often a patient) does not recognize or would deny. It's a subconscious motivation.
What we can say about transitioners is that they believe that they will be happier or will have a better life if they transition. The life they imagine for themselves after transitioning is better than the life they see for themselves without transition.
I cannot imagine someone saying, "I really want to win a gold medal, but I'll never do it as a man, so I'm going to become a woman." On the other hand, it would be folly to assume that someone struggling with a sexual identity, and who is also a competitive athlete, would not feel some sort of influence.
Let's put it this way. Let's suppose someone is "really trans", whatever that means. They are a high school, male, athlete, and they love it, but they live in one of those states where they just passed a law prohibiting males from participating in girls' sports. They know that if they chemically transition, which they are inclined to do, they will not be competitive as a male anymore, and they will not be allowed to compete as a female. Is there any individual who would choose to avoid transition in order to remain eligible and competitive? I'm pretty sure there are.
What that means is that race results might influence a decision to transition.
The rest of the implications are left as an exercise for the reader.