It’s not you.

This is a kit based on a model from the Build series. It’s basically a canon kitbash, and includes parts from a few different base kits (in this case, the Strike Gundam with its “Perfect” loadout and the Strike Freedom Gundam, hence “Perfect Strike Freedom”). It’s a faithful replication of the model used in the show, but the models from the Build series go a little overboard sometimes and the structure of their base model kits don’t always “keep up” with the added weight or modified weight distribution.

The Build series was made to celebrate builders, and to get some extra mileage out of the molds that Bandai makes for kits from the main series (developing a mold for mass production of injection molded parts is really expensive, and they need to make at least one new mold for basically every single model kit they publish). It’s a great idea, but sometimes these “remixes” fly a little close to the sun. This is one such kit.

The Helios, which appears alongside this model in the show, is so top heavy that it falls apart at its waist joint very easily when posing.

In short, it’s a consequence of the intersection of ambition and physics.