I’ve seen this question before (used in a meme) but that’s a curious one.

The answer is because they want to terrorize everyone else with their believes. They hate themselves as they’re are originally (what they would call “assigned gender/sex”) so they force others to participate in their delusions.

The answer why they can’t accept their biological sex is varied (many people have different stories) but what looks like shared by them all is: because they associate said biological sex with bad stuff. For example, a woman might think being a woman makes her only a sex object and a victim of patriarchy so the “solution” is to become a man or enby. A man might feel he doesn’t relate to the less “sophisticated” and more brutal/stupid men so he will become more feminine... At the end of the day, they reinforce all the possible stereotypes if they claim it’s otherwise and make life harder for those who live their own life and don’t fit sexist ideas.

In some cases, this is also purely a fixation on aesthetic aspect. Like associating being a man/woman with particular style and wanting to emulate it so bad that the body and identity needs to be changed. Then there are also people who had other problems related to body perception (such as self-harm, anorexia, experience of sexual abuse, being shamed by family members or someone else...).

Accepting trans identities will never help the people with issues to resolve their problems and instead will give a pass for horrifying perverts and accepting and hiding abuse and sexism.