CG movies are created in 3D, they use 3D modeling software. There's nothing to "convert" to 3D unless they lose the original models and have to retrace them or something. Which apparently is a quite common occurrence, according to some articles on the 2D to 3D conversion of Toy Story and Shrek. So collateral damage from capitalism is the main explanation for old movies.
For new movies, where there is proper data, the 3D camera for each scene still requires more than "zero human labor"; you have to specify the inter-ocular distance, convergence plane, and tweak the scene a little for cinematic effect or 3D weirdness. And since there are thousands of shots, and the work is fairly tedious, it makes little sense to work on it in parallel with 2D during production when shots might just be cut entirely. So you end up with a bias towards 2D or 3D during the creation process.
Dreamsworks apparently has a made-for-3D animated movie division (https://animatedviews.com/2007/dreamworks-goes-3d/), where presumably they are flattened to 2D in post. Reviews of their movies (https://www.3dor2d.com/reviews/how-train-dragon-hidden-world...) unsurprisingly say "great use of 3D". Since that's the goal of the division I'd expect them to continue doing 3D-first movies.
In contrast Pixar so far has stuck to a more 2D style. In this interview (https://www.youtube.com/watch?v=HepIGDJK98s) their stereo guy says they do 3D in post. The 3D in their movies is described as "well done but a minor aspect" (https://www.3dor2d.com/reviews/Toy-Story-4-3D-3-D-Movie-Revi...). In the interview the guy basically says that he could use more 3D but he's more concerned with keeping the director happy so he doesn't. Pixar is now focusing on originals so maybe one of their next films will be 3D oriented. But they're making tons of money anyway so who knows if the right director will come along.