Does no one understand what "main" means?

You don't need to play the champion every match, since then that'd be something else (one trick pony), but if you've only played a couple matches with a champion, or you play numerous other champions more than said champion, they're not one of your "mains." If we defined "main" as loosely as most people I see use the term, and we applied the same logic to movies, every character in most movies would be a "main character."

"Main" is meant to signify something more than just "I played this champion a dozen times." Depending upon the person and the champion, it can be quite a while before you really learn the ins and outs of a champion. For example, I've played a grand total of 120 matches with Ekko, tied for fourth most of all the champions I've played, and I'm still learning. That's also partly because he's weak now (except for in the most skilled hands) and I hadn't played him in normals in months before this week... but the point still stands.