“It’s almost like someone who didn’t grow up frum saying “When did this Shabbos thing start, it didn’t exist when I was growing up?”
No, with all due respect, it really is not. Shabbos is one of the Aseres HaDibros. Shmiras Negiah is not as beferush in the Written Torah. The W”T assers certain UNIONS between specific individuals, most notably incest and adultery. But it does not spell out what forms of contact are assur beyond “ish ki yishkav…” and then just lists the people who may not cohabit.
Yet the punishment for a man who seduces a virgin who is not halachically engaged (me-u-reses) to another man (as opposed to “engaged” as we understand the concept today), is far less severe than for one who commits adultery or has relations with a girl who is halachically engaged (as opposed to what we call “engaged” today). These halachos are spelled out in the Gemarah, but they are certainly not clear from the Torah that we lein, whereas Shabbos is upfront a mitzvah of zachor v’shamor. yes, we learn exactly what that entails from the Oral Law, but we know from the outset that keeping Shabbos IS the law.
By negiah, we are not so certain from the W”T what that means, only that certain physical relations are assur. I don’t know if I am expressing myself well here, but I feel it is a subtle difference.