Levels 14 and higher are trash levels. It's tolerable at first, but it eventually gets pretty unpleasant to DM and play. Level 20 has capstones to distract you from how godawful most of the non-spellcaster levels are at these levels, usually even compared to level 1-5 for those classes. Magic is way too good at these levels, but the desire to have those spells still in the game means they still exist. Encounters are difficult to run at this level, and and adventures are often difficult to plan. These levels are short to rush the PCs to the end of the campaign and save the DM's sanity.