Because the loon believes the 2nd coming of Christ already happened in 70 AD.
Of course, if the 2nd coming came in 70 AD, there would be no more need for teaching. But since those with such an eschatology teach, what do they really believe? They aren’t that foolish, but arrogance does account for much.
That is not a fair and accurate portrayal of what preterists believe. Most mainstream preterists believe in a literal future second coming of Christ. R.C. Sproul, who is a preterist, argues that to deny that essential of the faith makes one a heretic.
If we want to go down this road, let's take a look at extreme hyper-dispensationalists (John Hagee comes to mind with his denial of the need for Jews to have saving faith in Christ) and use them to paint the entire dispensational movement.