XIETIAN
It’s probably that the manipulation skills have power over all things of its type even stuff generated by other skills so if someone else can make gravity or blood the person with manipulation is like great more stuff for me to use. I mean if you think about MC with gravity manipulation has control over one of the fundamental forces of reality if you also count blood manipulation considering most complex life forms seem to have some type of blood.
The way the higher existences are reacting makes me wonder how the guy who first got the gravity manipulation skill get skill. I mean they say it's impossible and if the skill was given to bai Zemin directly you could say something like he is the reincarnation of something but that guy was ordinary and Bai Zemin just killed him to get the skill