Because it is something they felt the need to repress for a potentially long portion of their life. Coming out is the start of a process to release the pressure

Because it is something they felt the need to repress for a potentially long portion of their life. Coming out is the start of a process to release the pressure that has built up from that repression and letting the person's personal view of themselves realign with the public view of themselves. Also, yes it is weird that sexuality and identity are intertwined, but that is not something that is new and it is something that is tied to all sexualities, including heterosexuals.