For the same reason you like brocolli and I don't, and I like carrots and you don't.
I could try brocolli even though it's not what I like, but some how, I would not be happy, because it's not who I am. I choose to not like broccoli, however, it's because of how my body and mind is made that I am like that.
You could try carrots, but even if you had to fake you like carrots because people tell you that you should, you would just not feel right.
People make the choice to be how they are, this does not mean it's only choice that makes us who we are. You can either lie to yourself or accept how you are because of the way your body/mind was made.
This question can have answers forever, and follow up with a lot of arguing.
A scientist got interested in that question a decade or more ago and did some comparative autopsies on men who self-identified as gay and as straight, and (if I remember correctly) actually found some differences in a part of the brain (again, it's been a long time since I read about this research, but I think it might have been in the hypothalmus). Whether or not it has been entirely pinned down, there is agreement in the scientific community that it is inherent (that is, something someone is born with) rather than learned or taught. (Given how violently being gay is reacted to in some societies, if it *were* learned or a choice, nobody there would be gay, and still a given percent of people are.) Apparently, who one resonates to sexually (male or female) is more like a scale of 0 to 10, with 0 being absolutely straight and 10 being absolutely gay, and a whole lot of people falling in the middle or at least not a massive number at the absolute ends of the scale. That would explain why someone might find someone of the same sex attractive, but happily stay heterosexual in their activities for their whole life.