Because that's
not simple when you have to swap
twice within 3-4 GCDs at a GCD length that doesn't allow for most players to double-weave even when in merely FoF. Again, you'd be getting only 2 embonused oGCDs out of the bargain, at most. Maybe you're playing at 10 ping and zero bonus SkS because you despise everything to do with the Double-True rotation, but for anyone else that's just not a viable use of resources, not because the effects of the stances are so little but because of the gap-uptime punishments for swapping at all.
By comparison, if even just
one of those stance-swaps are dropped, such as during a one-tick Anatman opener, FoF is used for all oGCDs. That's all it takes, one oGCD gap less of penalty.
By extension, when the
penalties for doing so are less,
no oGCDs need be wasted, and the method of pulling off the technique
is far more convenient, one would expect it would be wholly usable.
What you're saying amounts to much the same as claiming that TK wasn't viable in late StB. You're completely ignoring the contexts by which its costs were so greatly reduced, or, in this case, so much of the costs and annoyances being removed entirely.