Because it's a risk most people intuitively understand, but most of them also don't know how difficult it is to "build a bomb" or "make a bioweapon".

In reality, the skills needed are pretty basic, but they overlap pretty strongly with being sane and well-adjusted. And if you are, you're probably not daydreaming about mass murder. Exceptions happen, Unabomber and so on, but they're pretty rare. In any case, Unabomber probably didn't need a tutorial.

We don't want ChatGPT to become an enabler and a co-conspirator for an unhinged person, but I think the concern is overdone.

Well that, and the average amount of easily obtainable explosives is substantially less dangerous then renting a box truck and crashing it into a crowd of people.

People go for conventional "exciting" threats rather then boring ones.