idk, people throw money into similar things in the real world, like the amount of money spent on professional racing. Or, you get things like robot wars where built robots destroy each other. I believe that people would play the expensive Gunpla hobby if it existed. Not everyone has the money, but enough people do.
Hell, people GAMBLE and lose thousands of dollars for a temporary thrill. That makes no sense either, but casinos make a good profit exploiting that nonsensical behavior. Imagine if gambling didn't exist, an anime about gambling would be seen as something that doesn't make sense since people throw away thousands of dollars on a game, then do it again and again. There's prize money in the Gunpla tournament too, so it could be viewed as a type of gamble.
Most of the focus is on a world championship tournament though, so it could be argued we're not seeing the "average" game of Gunpla as well, rather the few people with the time and resources to go world-level, we do see regular people battling or training with off-the shelf gunpla at several points in the show.
So, I've found a really cute romance manga called Girl Friends. It's this really charming slice of life story. I figured I'd tell people about it since it seems a little unknown.
De ja vu. Didn't someone have this same comment a few pages back?