Same reason straight women are often very harsh and backstabbing with each other, or Christians judge each other in directvcontradiction of their own scriptures that say not to, or poor people become shallow and materialistic, or Black people adopt White standards of beauty to judge each other by.
Anyone here remember when Gabby Douglas became the first black American Olympic women's all-around gymnastics champion! Immediately in the wake of that accomplishment, it was some Black American woman who went off the deep end publicly slamming her for "disgracing" THEM for having hair they deemed too nappy and not up to their standards. Total bitchiness!!! And yes, I know that word offends some self apoointed p.c. police. Deal with it.

The reason, in a word, is insecurity.

Vice is such a great channel.

Thanks for the enlightenment

The same is true in every group.
No one screws with Christians like Christians of the same denomination.

Try being trans, then you have truscum, also known as transmedicalist and transfundamentalist, are trans people who delegitimize the gender of other trans people. They claim one can't truly be transgender unless one experiences gender dysphoria, in this context it can sometimes be called physical dysphoria, as defined by cisnormative psychiatry and medicine.

Due to the fact that truscum are overwhelmingly a social media phenomenon, with a less than rigorously defined ideology, there are as of yet no academic or scholarly references to support claims about their demographics and beliefs. Notwithstanding, some attempts have been made to quantify their identities with respect to gender, age, and race. These results, along with their behaviour and views, lend support to the view that the typical member of their community is an adolescent USAmerican White trans man with little to no experience with the medical establishment, and has been out for a relatively short amount of time. Placing emphasis on dysphoria (often one's own dysphoria) in order to invalidate non-binary identities is often framed as an attempt to validate binary trans identities at their expense.

And that's just one of many pains trans people endure, from other trans people.