I always saw it likes this: it takes a toll on your body and aether to master a high-end job. Imagine learning several martial art techniques, it just takes time, practice and hard work. That is why, usually, people focus on one while the ones who are gifted or are really hard workers can double tkme it with 2 jobs. WoL, however, is an adventurer of whole another level so this is why they go, "20th job? Alright, let me master it in about a week."