Because of few strings in Koran about holy war with people who believe in other things. They can't understand that not everyone want to go at war.
It is not a "few" strings. There are more than 100 verses in the Quran (and even more in the hadiths), which urge the believers to wage holy war against the non-believers. And as far as I know, none of the Islamic scholars have said anything against these verses.