The same reason anyone would be: because their current body doesn't match their gender identity and they want to change that. This person just happened to start inside the same arbitrary social category as their destination.
The word cisgender (often shortened to cis; sometimes cissexual) describes a person whose gender identity corresponds to their sex assigned at birth, i.e., someone who is not transgender.
If they're taking hrt it sounds like they'd want to change their gender
Just as some AMAB (Assigned Male At Birth) men want to look more masculine and will work out at the gym or take testosterone supplements, some AMAB men are femboys and may temporarily take feminizing HRT to look less masculine.
Both are trying to change their bodies to better fit their gender identity, and femboy is clearly a different identity from gym bro, but they are both male gender identities.
In short: Femboy is about presenting as a very feminine man, men doing that can include trans men.
Slightly longer: It's why there are different categories for sex, gender assigned at birth, gender identity and gender expression. A femboy would identify male and express feminine. The opposite is a tomboy, somebody who identifies as female and expresses in a masculine manner, both are not necessarily linked to the individuals sex and/or gender assigned at birth.
Trans refers to a different gender identity than the gender assigned at birth and doesn't come into play here.