It's because there isn't actually confusion about this. This is transphobes making up something to be angry and confused about in order to rope in the ignorant to harass trans people. It's not acceptable to say "trans people are bad, we should ostracize them" currently. So transphobes find something that could be confusing (nonbinary people using they/them) and convince ignorant people (people who don't know much about trans people and/or have no opinion) that it's confusing and wrong and people should "correct" them. Then you get ignorant people saying things like "they isn't singular" or "I can't get used to they/them and don't like using it." This creates a continuous debate on if trans people deserve to self-identify and generates constant micro-aggressions (or just full aggressions) against their entire community.
It's really just a way for transphobes to create a hostile environment for trans people over literally nothing.