Because femboys aren't under the delusion that they're hot sexy women. They realize that they're men who happen to like being feminine, so therefore, they're more aware of what they need to hide, coverup, or do in order to look more convincing. Also, most femboys happen to be androgynous looking already, so therefore it makes it easier for them to look convincing. femboys also study actual women to learn how to be as convincing as possible, whereas troons get their ideas from other troons, or what they see in anime and porn. Also, most troons are obviously masculine men, which is impossible to hide, no matter what kind of surgeries you get.
Back in the days when there were strict guidelines on treating dysphoria, one of the criteria of being allowed HRT and surgeries was that you had to be able to pass convincingly as the opposite sex. Those who were deemed unable to pass were turned down for transitioning, because psychs figured it'd just make their dysphoria worse when people wouldn't recognize them as the sex they wanted to be seen as. Also, you used to have to have intense therapy and make an effort to live as the opposite sex before they'd hand over the titty skittles and chop off your dick, because they realized that these surgeries are permanent and long term effects of HRT were unknown, and there is no going back if you change your mind.