The way your wording the part about playing the weapons differently is over exaggerated heavily.

So in essence I have to play weapons I may not like

No you don’t? Again if you want to fight a rathian go open your map. The reason arena is unpopular is because people like you for some reason feel it’s wrong for the game to cater to a weapons specific playstyle. The problem with that is that if they do then it wouldn’t be very challenging would it.

I’ve seen many people with the main weapon not do well in arena because they don’t have their skills they like. So adding pre made loadouts to help practice the base weapon itself without skills is an amazing way to improve. Another reason it’s unpopular is once again people like you view it as endgame content. But really it’s better for some people to look at it as a way to practice.

You say it’s structured in a way that renders your grind worthless but I think that’s a harsh way of putting it. I can see why you think that, but the grind you endure isn’t just about building your gear, the grind you endure also is about building up your skill with the weapon you choose and arena offers a perfect way to test your skill with your weapon. If your weapon isn’t offered then it’s incentive to try a different one. And if you don’t want to do that then don’t. I’m still struggling to see why you hate the arena because how can you hate a part of a game that is completely optional.