Hello there!
Probably because not all of us can be a jack of all trades. You can be a writer, but know nothing about programming or drawing, and vice versa. Honestly, I'm a shitty artist and you can tell if you play one of my very first games, Clock Manor, where I drew everything by hand. I actually work alone on passion and giving credits to artists is the best thing I can do. Also today we are all witnessing the emergence of new AI technologies that allow us to generate art, music, voices and even videos, and this has undoubtedly seriously affected both artists and developers of adult games. I've seen commercial and non-commercial games using AI 18+ art. Every day it becomes more and more difficult to distinguish what was drawn by a person and what by a machine. I wonder how digital art will develop further.