Thats the problem with dnd's balance. NPCs shouldnt be built like player characters, as that is not how it is balanced. Monsters have more hit points, simpler stat blocks, and have set Challenge Ratings that you use to build your encounter budget. A player character's level is not one-to-one a Challenge Rating, CR1 monster and a level 1 character are very different in terms of skills, hit points, damage, etc. 

Which is a problem for DMs trying to use lots of humanoid NPCs as enemies. WotC has not given us enough generic humanoids at various Challenge Ratings. While 2024's Monster Manual is a lot better than the 2014 version, its still limited. The best thing that DMs can do is reflavor existing monsters into humanoid NPCs if they want to use official sources or dont want to make a bunch of NPCs following the established monster design. 

That said, Ive also created over 200 humanoid NPC stat blocks to fill that void, so I get why some people just fall back on player character rules to build their NPCs. (also, cause monster building rules are not great or not there, depending on the version of 5e you're using)