Its probably a combination of poor quality metal, and poor tolerances. Chinese manufacturers aren't known (at least in my book) for spending a lot of time or money on small parts that aren't necessarily critical. If it can be installed once, it's good enough.

Just another reason a HL doesn't cost what a Tamiya does.

I had a crossthreaded screw into one of the motors on my Panzer III, but hey, it was like $140!