They are born from Sephiroth's will and embody different traits of him. Kadaj is Sephiroth's cruelty and insanity. Loz is Sephiroth's Strength and speed. Yazoo is Sephiroth's calmness and stoicness. Together they could become Sephiroth. His will power allowed him to "live" as his remnents instead of being pulled into the lifestream. Hope that answered your question. If you don't believe me go look it up.
Also Due to JENOVA's cell's in the lifestream (Geostigma) aswell as the men from SOLDIER were experimented on and had JENOVA cells injected into them, they became more like Sephiroth, but none had been such a "perfect" specimen as he had. So It's all due to JENOVA and her/it's cell's.