That's like saying why not translate 'kurenai' as 'scarlet' or 'yomigaeri' as 'the resurrected' or 'hiryuu' as 'flying dragons'. That's what they call/refer themselves and so I'm sticking with it instead of a generic word (even though the Japanese words are generic themselves).
For the first chapter, I did translated it to 'guard' but after translating this chapter, I felt calling them that or 'garrison' or anything else didn't work as well as 'keibitai'.