why not just buy Chaos Daemons and slap them on square bases?
Because i don't like Chaos, i've played a Chaos fantasy army and was not impressed. it didn't fit me, or my play style. an army is much than the sum of it's models, it has a soul as well, and that soul is you. your effort is what makes the paint bring the model to life, the conversion and little attention to details that you add are what makes the model unique, in effect giving it a spirit, and eventually the entire army comes to life under your ministrations, and you become a part of it throughout the whole process, in effect becoming it's soul. if an army doesn't fit your play style, nothing you can do will bring that army to life.
at heart i'm a tech head, so Eldar with their grav tanks, Necron with their machine bodies, Orks with their cobbled together vehicles and cybernetics, and Skaven for the warp tech all fit my play style and speak to me as a tech head. my nids even do the same thing, because even they are a tech army, it's just that their tech is organic rather than machine.
and now I'll stop with the Buddhist Tao speech.