The reason is so a player doesn't get lucky at Paragon 1 and magically find a Primal Ancient that propels them so much further and faster than everyone else. Even at a 1% drop rate, 1% of the player population will find one day 1, and a good percentage of those will be weapons.
Yes, we see day 1 lv 70s and day 1 Paragon 200's, but locking behind the GR70 makes sense at least from the season race perspective. It smooths out the curve a bit in power acquisition.
I'm still disappointed that Primals only yield what amounts to a small stat buff. They could have been so much more. There's simply no chase factor on these for me.