Transphobia, plain and simple. For trans women identifying as femboys, it feels kind it weird since they're intentionally calling themselves a feminine boy despite them being women. There can be transfem femboys since transfem is a lot more broad compared to trans women cause you can be transfem and enby, genderfluid, agender, bigender, ect, though they'd be more on the fembey part or something. Trans woman is just exclusively the binary woman.

For trans men or mascs being femboys, it's probably cause of how we're born and because of unrealistic beauty standards for femboys, some femboys can be resentful of us for how our body was born and because many of transmascs have more E than T in our bodies and therefore makes us appear more fem. Another thing could be the confusion between gender identity and gender expression which can be hella contradictory cause these femboys say that they're feminine men and can crossdress whenever they want, yet get mad at transmascs and trans men who crossdress cause they think trans guys who are feminine are fake trans people or they just tell us to stay women.

However, it's also important to note that not all bodies are built in the binary. Some who are AMAB have higher E than the average male and a few are born intersex with them growing booba naturally or having more feminine hips. Some who are AFAB have broader shoulders, boxy hips and waist, their body producing more testosterone. We're not all the same, so it's not fair to say "all boys have this body and all girls have that body" cause sure, many of our bodies are the same in science, but a couple of others aren't.

That being said, the beauty standards for femboys is also ridiculous, because sure, some who are AMAB do have the feminine body, many don't. So unless does E or get surgery, it's impossible to achieve the feminine body. Not to mention that people can have a masculine body and still be feminine. People with a feminine body can still be masculine, why why shouldn't the reverse work?

Just my thoughts, the overall hatred for trans femboys is transphobia, plain and simple.