One problem it has is that it can be REALLY slow sometimes. Like any turn based strategy game, map size has an exponential effect on how long a game takes, but the mechanics of wesnoth lean a lot more towards larger maps.
I’m mostly talking about multiplayer though. For singleplayer there’s about a hundred million campaigns to play so you could probably play it forever.
I think the bigger factor is, someone who already thinks they’re great probably isn’t working on noticing and improving their weaknesses. Someone who thinks they still have a lot to learn is putting a lot of effort into improving.
So, especially if they’ve felt that way for any significant length of time, it’s no wonder which person will end up being better.