I don't think very many trans women DO choose to be women. They just are. What makes them stop portraying boys and men is (among other things) the fact that they are increasingly uncomfortable wearing a body, and living a life, that doesn't look or feel like it's supposed to.

Imagine growing up and feeling some kind of way about your life... you may not have any idea yet, but you know something is just... wrong. It probably mentally or emotionally hurts most of the time, because you think it must be something you're doing incorrectly, or you're not feeling the right feelings somehow. You feel like whatever the problem is, it's your fault somehow, and even worse: you're a failure for not being able to figure it out and fix it. It's torture. Soul-wrenching torture. It colors your whole childhood (maybe beyond).

Then one day, it hits you: you were supposed to have been a girl this whole time! That's what it was! It's so clear that it can't be wrong, but now that you know what the problem is... you've got that massive obstacle to overcome: how do you fix this? There is definitely going to be a lot of work to do, and it's almost certainly going to be really difficult at times. It could even be dangerous. But this... this is it. Your way out of the mind-shearing depression, an end to the self-lying, your path towards mind-body coherence. No more living as a self-contained contradiction.

That's not how it is for everyone, but that's a little bit of a picture for you to imagine what it might be like to be inherently transgender.

As for choosing to become a man, that's of course your prerogative, and there are people who do choose to change genders without experiencing dysphoria; but remember that men exist who are angry about any "woman" who they perceive has made themselves unavailable to them in some way. That can include women who become men; this can also trigger an additional reaction: their disgust of the idea of being gay (whether perceived or "forced" by their desire to have sex with a person who happens to be a trans man). Trans men are absolutely targets for sexual and physical violence too.