Because it raises the bar of entry to exercise a right afforded to you by the constitution.

Because 2A-ers see it as a right to use a tool, and like most tools they already come with the appropriate manuals, warnings, storage/safety recommendations and safety guides.

Because it could become a part of the "death by 1000 cuts" that pro-2A Americans fear by becoming too lengthy, expensive, inconvenient to find/take.

Because most pro-2A'ers learn about safety, local, state and federal rules and common practice through others it's preached as much as their disapproval for the latest piece of legislation trying to shut down their local range, threaten to make their possessions illegal and break up their community

Because states rights: what might be a well intended idea in New Jersey (say, how to recognize and avoid becoming a target of gang violence for what's on your hip in Newark, doesn't apply to the farmer in Montana)

Because someone likely to commit a crime will find a firearm on the black market or via some sort of straw purchase, not legally take classes and buy/Use something that's traceable back to them

And because there's not much to teach besides a refresher on societal common sense. Things we can and should educate everyone on, regardless of which side of the 2A debate we side with