Level playing fields for women and girls aren't served by allowing competition from people who haven't always been female. Its not fair on cis women to have to compete against people who've had advantages from going through puberty while male.
The purpose of women's sport isn't to be inclusive of women, its to be exclusive of men. And its not that it's too much work to include some trans women on the basis of ability, it's that it's just impossible. Do they include only those who aren't likely to win? Maybe some that can win, but not by too much? What about a champion male who's recently transitioned and would shatter the world record, making it unattainable for any cis woman for years to come, maybe ever? There's no way of making those judgements, no matter how much work is done.
Its the same principle as banning performance enhancing drugs. Some clean athletes might beat some drug using athletes, but we don't try to figure that out, we just ban drugs. And puberty as a male is getting a few years if hormone-induced muscle gain that isn't fully lost even post-transition, even on hormone blocking drugs.