I have thought about this and have compiled an answer to this question.
It probably has something to do with a "cute factor" that most femboys tend to have, as most tend to be submissive and fragile in terms of appearance. This invokes paternal or maternal instincts within the mind of the influenced, triggering impulses to embrace the femboy as if it were their own child.
Obviously, there's also the feminine appearance of the femboy in question which makes them attractive to a potential partner. Femboys tend to be short and slender, lacking any facial hair or muscles as well as a higher pitched voice. They also tend to wear feminine clothing and any other apparel that highlights their stomachs, hips, or thighs and hides signs of masculinity.
Another factor to consider is the person in question who finds femboys attractive. Most of the time, the influenced shows attraction to males, and finds the femboy's fragile appearance and submissive demeanor alluring. This can come from a strong sense of dominance or a desire to care for their partner, depending on the personality of the influenced.
And that's what I think makes other people attracted to femboys. If you believe there are any errors in my assessment, please let me know.
Have a day