
to answer the original question, people are against it because people wouldn’t like to date someone who’s still friends with their exes. to be friends mean to text, hang out, etc. that’s uncomfortable as a partner because the two of you had feelings for each other and even had sex together.
also, to answer this question, it’s because not every initial feeling you have for someone is platonic. sometimes, there are people that you meet that you immediately see as a romantic interest. so then then you date, then you become a couple. it’s completely possible (and not weird) to not be friends first.