People have stupid opinions because they base their thought on emotion and not logic (despite their assertion otherwise), do not take the time to understand the subtleties thar should inform their opinion or....and this is a biggy, the simply do not want to change. They do not wish to have the cognitive dissonance of a new or differing idea. They've come to a comfortable spot in their ideas and no reason to change. While I wouldn't characterize my sister as having stupids ideas, she has come to a point in her life where she would prefer to believe something that is wrong than to invest the time in understanding why a different opinion or approach may be right. Fortunately, this applies to some pretty harmless stuff but if you start edging in to that uncomfortable territory, you are greeted with the technique of off-putting I call 'I know, I know'...where she says the affirmative words but what she's really saying is 'Stop saying that, it's make me uncomfortable.' Key is to face the persons unwillingness to address the facts. Most people that are greeted with a logical argument aren't always going to argue them but try to avoid them.
