They're probably the reason. OnePlus was designed for a pure performance standpoint. Flagship performance at high mid range pricing. Something has got to give for the company to still turn up profits. Like screens, sensors, etc.
Basically I personally look at OnePlus phones as taking a high mid range device like Galaxy A or LG Q and then putting flagship SoCs and RAM in them. It's going to perform as fast or even faster than the Galaxy S/Notr or LG G/V, but it's other things like screen quality or cameras may not come up to par.