The LGV route is a far more open and "flatter" (relatively speaking) than LA to SF. There's way less for them to tunnel or grade, which gets expensive quickly. HSR requires more gentle changes in direction than traditional rail. Building on flat, open terrain that no one is using is great... but that kind of terrain is already in use or owned by someone. Thus, easements or having to outright purchase the land. If not and you go for cheap land, it's going to come with downsides, like being mountainous.
At that, France sees less than 10 earthquake events over 2 magnitude a year on average. In the past week, there were already 6 events over 2 magnitude between LA and SF alone: https://www.cisn.org/map/index.html An LA to SF line is practically on a fault line. Devastating earthquakes are a real reality for a Cali line, not so much for France. The Cali standards for building rail are going to be higher than France, and for EXTREMELY good reason. Thus, price goes up for construction.
There's a lot more at play to building these things. What I mentioned barely scratches the surface to the complications. Claiming NIMBY is pretty naive if you don't take into consideration the real world problems and needs to achieving something compared to the lofty, idealistic wants of a project.