Because we do not control for every form of "cheating" if we expand the definition of "cheating" to ridiculous extremes, does not mean that we should just abandon all enforcement. If everyone is cheating, then no one is cheating, right?
In the name of fairness, we control for what we can. Age separation. Sex separation. Skill separation. In some sports, weight separation.
In only one category today can you change where you are placed with just a word, overnight.
You are the one changing the definition of cheating, and also seem to be under the misunderstanding that "cheating" means "not doing what I want". Trans girls who participate in sports with and against other girls are doing so in accordance with both the law and the league rules no matter how much it bothers you.