Most plastic coffee drippers are manufactured by Hario Glass Ltd, Melitta, Clever and others, The advantage is that they are cheaper than other materials, and in the case of Melitta, available in almost any super market.
Plastic is not considered inert, it can effect flavor depending on material, although resin is more glass like.
Plastic is not bad and most people would not notice any flavor effects. It’s cheap, and Hario makes a version of it cone dripper in glass, ceramic or glass. So if your on a budget and want great pour-over coffee, plastic will serve you nicely.