Cheap devices get subsidized by shitty adware. The cheaper the device, the more likely that it's full of terrible adware.

Consumers often have a choice, at least between "filled to the brim with crap" and "a modicum of crap", by choosing between buying their phone from a store or from a carrier. Carriers have better deals but shovel their phones full of the worst apps you can imagine. Still, people will buy the crap-filled experience that makes you want to tear your hair out because they like the idea of scoring a better deal.

Nothing like unadulterated greed combined with short-sighed consumer behaviour at scale to drive a market segment into an awful race to the bottom.