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.