>>937458050 (OP)technically, all humans have the anatomical structures we call breasts, but I take it that you're asking about chest structures that resemble those of adult human females, yes?
some "boys" could have for many reasons:
they're obese
hormonal issues
taking dedication to cross-dressing to the next level
they want to feel like a woman
the "boy" is a mentally ill woman who has yet to chop them off
drugs
a mentally ill man/boy/thing
they lost the genetic lottery
they "won" the genetic lottery
they forgot that pp goes in front butt not butt butt
and probably others I can't think of