HRT may be a lifelong process, but most people don't start it early enough to prevent the development of secondary sex characteristics and muscle structure.
For FtM: Part of the reason that you don't let men who take testosterone compete is because that would give them excess testosterone and a competitive advantage. That competitive advantage seems to diminish when you grew up female and are transitioning to male.
For MtF: The muscle structure of a man is biologically different from that of a woman. It's not a tricky line, it's an unfair line. However, it's one that needs to be drawn. Any woman who was caught taking testosterone supplements would be banned from competing in women's sports. Somehow all that excess muscle structure is overlooked when a man is transitioning because then it's politically incorrect. Unfortunately, HRT is a very gradual and slow change. It can take years before you even begin to see the kinds of changes required to level the playing field. The fact is they were born to a gender that in some sports, has a biological advantage. To ignore that for any reason is a disservice to all the women who train to compete at the higher levels fairly.
Oddly enough, sports isn't about equality. It's about inequality. One person being better at something than another because they were born with better genetics for that task.
as part of transition, many trans individuals go through a lifelong process called Hormone Replacement Therapy in which they take medications which block the production of their sex hormones (testosterone for people born male, estrogen for people born female) and then start regularly taking the other hormone either by pill, patch, or injection.
for FtM (female to male) individuals, most find this promotes muscle growth. it's essentially just taking steroids. for MtF (male to female) people, most find that significant muscle mass actually deteriorates and they end up being physically weaker.
does this put FtM and MtF individuals on par with cis people of their gender identity? probably not fully, but they're definitely not on par with their biological sex either.
you wouldn't allow women regularly taking steroids to compete in women's sports, would you? because that's FtM people competing in women's sports would essentially be. but with the flip side, MtF, we grapple with where exactly to draw the line.
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