Sam, most of who? Not Rabbanim Gedolei Yisroel throughout the generations from his time. I don’t either think Rebbi Shimon wrote it, just like Rebbe Eliezer didn’t write Pirkei D’Rebbe Eliezer. Neither did Rav Ashi write the Gemara. Most of these Sefarim were written by Rabban Savurai or early Geonim.
The Ramban had the Sefer Habahir and a Kabbalah from his Rebbe. As I said, what you find from beforehand pales in comparison to the Zohar Hakkadosh. R’ Moshe De Leon was no greater Mekubal than the Ramban. The Ramban writes that we don’t have the Sod of Arayos, which is in the Zohar. It is very obvious from many areas that the Ramban was not privy to the Zohar. In fact, before it came out, Rebbe Shimon Bar Yochay was not the central figure of Kabbalah as he is today.
Although to you this is all one mush, to those that learn these Sefarim Be’iyun there is no Shaychos between what preceeded the Zohar and what followed. This is besides the fact that more was found after his time.