Because it is not inherent. It is subject to the whim of fate.
Consider this reductio ad absurdum. A large merry-go-round constructed on the border between the U.S. and Canada. Pregnant women get on... we give it a spin... and no one knows what the citizenship of their children will be. Not so for the jus sanguinus principle.
Consider another reductio ad absurdum. A world wide matter transmitting network. A pregnant woman goes into the room, something goes wrong and she gets "beamed" to cities all around the world, one after the other. What nationality is her child? Like Schrodingers cat, we won't know till we open the box. Again, not so for the jus sanguinus principle.
The theory that one's citizenship is determined by place of birth is no different from suggesting that other inherent characteristics should be determined by place of birth. Birth in a stable makes you a horse. Birth in the water makes you a fish. Birth in a hen house makes you a chicken.
Absurd. Natural characteristics are not decided by man made artificial boundaries. They are inherent.
Your theory would yield the bizarre conclusion that John McCain, born in Panama while his parents were serving in the nation's interest, has not sufficient allegiance to be considered loyal enough to be President, while Barry, born to a foreign father (so they say) and raised for many years in a foreign country with a foreign culture (not even English, Aussie, or Canadian) with not demonstrably reliable proof that he *IS* even born inside the borders, is.
Jus Soli was created by the English kings to lay claim to anyone born on their soil. It was a device to grab and hold servants to their perpetual allegiance. It does not serve the needs of a free people. That the colonies all utilized British law prior to our independence accounts for the fact that it has persisted as an idea since this nation's founding, despite the fact that "Subjectitude" and the implications thereof, was specifically rejected in not ONE, but TWO wars with England. Jus Soli was later invoked in 1868 for the purpose of granting citizenship to former slaves, because the slaves could not claim a birthright through jus sanguinus.
It has ever since been consistently misused against the interests of this nation, as the relatively recent phenomena of a President with dubious allegiance and "anchor babies" illustrates.
But by your definition a person born in the UK of two U.S. citizen parents, for example, and then raised there for the next 21 years has more of a claim on the presidency than someone born in the U.S. and raised here their entire life but whose parent's happened to be non-citizens. That makes little sense to me. Or to Madison. Or Rawle. Or Kent. Or others.