Tau Caste is not a purely social construct as it would be in us humans, it has a basis in their biology. Imagine a human bemoaning the fact that he can't be a cat or a dog, that's the kind of thing I'm talking about here.
Wait, are we comparing Tau society to the real world or the rest of Warhammer 40,000 universe? The 40k universe where there is only war. The 40k universe in which without a dedicated military you can't dream of having a functioning society.
Yay, we're all free to do whatever we want! Oh no, Orks are attacking! Where are our soldiers? We don't have any, nobody wants to fight lol. Oh no, we're all dead.