Because it's easier than actually addressing their arguments.
End of story.
There are also those who believe lobbies are inherently evil, although they're nothing more than the organized, large-scale practice of representative democracy. Some people like to discount corporate influence on them, as though corporate interests are not legitimate in a capitalism-based society.
The truth of the matter is that if there is enough grassroots desire for something, no corporation or political entity can stop it. Look at the tobacco lobby. They were more powerful than almost any lobby, but when enough people wanted change, they couldn't stop it.
Same thing is true for gun control. If enough people really
felt the NRA was evil and had to be stopped, membership of 4.5 million with a $300 million budget would be pretty easy to overcome.