Well, to be fair, the Scout Trooper requires the widest range of motion to get it to sit on the bike and make other poses. It's asking a lot of rigid styrene to mimic flexible skin, muscle, and fabric. I mean, physics is physics. Frankly, given that the trooper looks as good as it does in a variety of poses is something of a minor miracle. There are videos on YouTube of a guy who has wrapped the open joints in fabric, and I'm sure there are other creative solutions to the problem. If nothing else, select a pose and fill in the gaps with putty. Personally, I don't want playability (my models sit on a shelf and look perdy) and I don't need much poseability (I usually find a pose I like and there it stays), so filling the gaps with putty is a good solution.
What's great about Bandai kits is that the detail and accuracy are so good, we can spend our time finding creative solutions to more interesting (and maybe more challenging) problems than we did during the decades when we were wrestling with the hugely inaccurate and out of proportion AMT and Revell kits. I once spent weeks on the old AMT Speederbike kit and used two kits to cut the trooper's armor into individual plates. The Bandai kit is basically the model I built right out of the box. Honestly, it's WAY better. I'd have happily spent a day or two filling joints rather than weeks carving and reconstructing basic components.