A lot of the challenge does come from the dungeons.
Grinding levels is pretty mundane, and not really difficult. It would be easy to just add a hundred levels, throw in a few monsters (or generate them automatically based on your level), and basically make an "endless mode", but that's not really the goal of the game.
Personally, I measure completion based on which dungeons, and which modes you cleared (this is also how you get most of the armour in the game).
This is another reason why levels are released more slowly - in addition to quests, levels are also tied to how many dungeons exist. For example, when I make a dungeon with a recommended level range of 39-X, I will wait with new levels until I added all the fights you should be able to complete at 39, so we can actually test them at that level. All the dungeons in the game have been cleared at the lower recommended level range they display.
Simply grinding for levels forgoes a large part of the game, which would be a pity. ;P A lot of new players have cleared the East Caves, the Hollow Tree, some Frigus and Casper, but I don't think I saw many with BFA (and beyond), despite having the level for it.