Why does a topic sentence have to be in the beginning of a paragraph?

It's typically to let the reader know what that paragraph is going to be about. However, I know I sometimes would prefer to organize a paragraph a different way. For example, I might let several apparently unrelated details precede the topic sentence, and then use the topic sentence to tie it all together. Unfortunately, most teachers throughout your school years will not think this is an okay thing to do. I have gotten several papers back throughout the years that said something about my lacking organization. Most of the time, it is the author's choice, although in school, you should do it the way the teacher wants it done. So, basically, there is nothing saying it has to be in the beginning, but you should, at least for assignments, projects, etc.