I mean, I've taken more than 490 hours of Spanish, but I don't speak it fluently, nor am I a professional translator. I've played far, far more than 490 hours of trombone, but I am still very much a novice in terms of musical theory and technical skill, when compared with the spectrum of competency.

Yes, repetition increases familiar and ability, but you can't just generalize and say that anyone should be "average" at anything, without first addressing the depth and dynamic nature of said interest.

You probably already know this, but DOTA2 has an extremely steep learning curve and an even more substantial skill ceiling. At ~490 hours in DOTA, this means one could have played maybe 4 games with each individual hero.

Many initial hours are spent learning the most basic aspects of gameplay, e.g. how to move, what buildings and units are actually doing, the logic behind map geography, how lvl/gold progression happens in a match, etc. It's a long time before you even start scratching the surface of what individual heroes' abilities do, let alone how they synergize with each other or why a character would effectively pair with or counter another given character.

Each hour you commit to the game involves reacting to the events in game, not necessarily furthering your knowledge of the plethora of items/recipes/builds, timing for stacking camps/runing/roshing, when/where to push/defend, etc. Each single hour session usually just offers experience with your personal hero and some increased familiarity with larger strategy.

Regardless of all the reasons mentioned above, the main factor that causes such a gradual understanding/mastery of DOTA is the Meta Game. Between the constantly shifting trends in strategy/counter strategy (influenced by top tier celebrity players) and the ever present updates, the game is absurdly dynamic. Imagine if chess had hundreds of more pieces and every few weeks, each piece was allowed to move in a different manner than before.

It's hard to believe, but definitely true that 490 hours wouldn't provide more than a "casual" competency in a game as deep as DOTA2