Jungle is the Most Grueling Role by Far

Front line champs are mostly utility carries and often lane, split push, engage, and carry well. Champs that flank also have to dive, time ults, peel, gank, etc. Meanwhile a lot of backline champs just have to hit some shots, position well and take good routes. A lot of people are OTPs too. So these players get to progress the same amount doing a lot less. Meanwhile jungle is very grueling. Every game you have to try to be everywhere, and do as much as possible, while often not being able to carry. Some of the load needs to be taken off the jungler, which probably amounts to less volatility in some lanes.