Because nobody writes many of these.
(Y/N) was a anti-social girl at the Galaxy Garrison, best in her class, not many friends, not that she cares, the only kids that ever hung out with her had hurt her, so when she got accepted into the Garrison, she had nothing keeping her from leaving. (Y/N) was sent to Kerberos with the Holts and Shiro, and was kidnapped with them, but she didn't get kept prisoner, she was forced to train to be a Galra soldier, she had lost all hope until one day she escapes.