This is complete speculation, but my guess is something like: the people who might understand this (skilled directors and such) are used to 2D and don't much care for 3D, and the people who push 3D (executives) are too obsessed with making things "pop" to realize what they're doing.
Either they are huge, to get two cameras side by side, or they have a half-silvered mirror arrangement (with colour disparity)
Add to that the rigs wobble (vomit inducing) and the distance between the cameras is far to wide, it all looks a bit poop, or requires a huge amount of post work to make fly.
So the normal thing to do is manually cut out each object (rotoscope) and adjust the divergence to place it in 3d.
every object, every frame.
it mostly looks a bit poop.
Not to mention is normally done quickly, like clash of the titans was converted in ~1 month.
