I'm unsure of the intricacies of Overwatch's rating system, but the problem I've noticed in most rating systems is that players don't play enough games for the rankings to stabilize. You end up with a situation where a very large amount of players end up close in ratings, where only a few wins or losses can propel them way higher or way lower.

This might not be a big deal, but the game isn't static. It changes, which means that the skills someone picks up might not be as useful in the next patch anymore. This keeps the rankings somewhat unstable.

Team games inherently require more games to stabilize a rating too. You can get lucky or unlucky with your teammates. On top of that, a game where you can end up with a character you're not strong with probably increases the number of games to stabilize a rating even further.

On average it might work out, but are enough games played to actually reach that? My guess is that for most players the answer is 'no'.