Whatever spoils you get from a level 30 event will just be replaced in 5-10 levels. Also, without a level cap, anyone can get high-level help, making the event easier. If the event is easier due to this, then more people will get the spoils. If more people have the spoils, then the market for anything level 30-40 will become a dead zone.
And again, if those 5 to 10 levels take time, it's still worth doing. Also, as I said, if the rewards would include, say, accessories that are useful even for capped characters (Such as things that offer bonuses to skills or are useful in certain end game content) then that content becomes relevant even for characters at endgame. Alternatively, the rewards don't have to be gear at all, instead something that provides a permanent utility to a character (Example, completing a dungeon would be required to activated an atheryte camp, giving the player faster travel as they complete content, or, say, being how you acquire a chocobo pass etc)

And still, I suggest a cap for mid game content regardless