Several reasons I think:

1. One of their tactics is to exaggerate their vulnerability. Words are "literal violence". Disagree with them and "they'll kill themselves". And when you point out common sense biological differences you're "spreading to a harmful ideology that contributes to the murders of the MOST VULNERABLE marginalized group of people". Throw a totally misleading report on the number of transwomen murdered each year. Now you got a brew of something along the line of literal murder by mere disagreement. Don't you care about their lives? They're human too!!!!

2. They appropriate mantras of other groups. Feminists used to have the mantra 'Women's Rights are Human Rights". That's where "Trans Rights are Human Rights" came from. They've also co-opted "Black Lives Matter" to "Trans Lives Matter". The feminist one is about rights and the black lives one is about saving lives (from police and other forms of violence). They then conflate the concepts of these two mantras and hence people like you looking from the outside are scratching your heads having cognitive dissonance.

3. Their entire existence depends on validation from other people so if you don't validate them and play along with their fantasies, they no longer exist.

Edit: changed a word for clarification