Why is it so common to see Americans using the term "vanilla" as a synonym to normal people or normal life?

I am mexican and the only time I see the term vanilla is when I'm eating a vanilla ice cream or a vanilla cake, so I don't really get why you guys (it's always the americans) are always so obsessed with the term "vanilla", like "oh, I want a vanilla relationship" or "i want to go back to my vanilla life" and stuff like that, like it doesn't make sense to me when you have normal synonyms like the ones I said, so I guess vanilla people will be called vanilla people. <br><br>But if you want to call me a vanilla boy, I will take it as an insult just for context ???