You are right. It has several reasons, nobody knows for sure what it is yet, but bottomline is: fillers have been proven to create this hyperangular look on a consistent basis. So, the question shouldn't be: why are there so many bloated cases with hard materials, but: how do we create a safe permanent filler mimicking results of temporary fillers.
There are two other things I want to mention:
1) People talk about designs when it comes to surgeons. But when you look at the designs from several surgeons, they are pretty much the same, globally. Design is one thing, but more important (probably) is how the soft tissue is layed around the implant during surgery and the surgical skills to do this.
2) When you look at the jawbone/structure of people with naturally good jaws, they never have the same structure as for example Eppley's designs. Sometimes, there's barely any difference between a ct from someone with a good jaw and someone with a bad jaw.
Conclusion: soft tissue and maybe masseter are much more important than we ever thought. A permanent filler is needed to recreate the hyperangular jawline, which implants almost never are able to create. Or, if it's possible to create it through hard material, a better material, and then we return to question 1: why do many implants create a bloated look? This is a tough question to answer. There's not much scientific research about it. My guess is: A) implants need to go up the face to be able to fix them, so they target areas of the face that shouldn't be thicker. For example, the area between the lower border of the jaw and the cheekbone should stay 'empty' in the ideal case. But implants go up to that area, creating a bloated and flat face without depth. B) They add mass to the bone. They add a layer on top of the bone. Same goes for chin implants. It looks a bit unnatural, the natural bone isn't that thick. C) phenotype might play a role in some cases as well. It's hard to go from oval to angular with hard material without making it look unnatural or bloated. D) jaw doesn't stand on its own. You have to look at it relative to cheekbones and chin.
That being said: some people have faces that are too small. Using hard material might not give them the superangularity, but could make them more aestethic looking, just by adding mass to the bone.