@dba_afish: I think it's because tomatoes are kinda sweet tasting. Closer to what people generally think of fruits as this sweet part of the plant that you can eat. Unlike most types of vegetables.

It's also because people first heard about most science "facts" like that when they're kids, and kids are extremely gullible and impressionable. And depending on how the school educates them, or even just how strict the teachers are, students might not even bother to question whether what they learn from the book is true. A lot of schools recently have adopted a more explorative kind of learning in their curriculum. But a lot also haven't.