People are drawing Deltarune so much not because they're obsessed fans, but because it's the only game with mass pop culture appeal that uses pixel art sprites in the last decade. Even Stardew Valley is 10 years old at this point. There's literally nothing else to draw unless they scrape from ancient games (from the perspective of most users) like SNES and Gameboy Advance games

More detailed and more realistic art is not necessarily better art.
There's no absolute measure for quality of art; and I don't just mean because taste is subjective, but rather because art in media can have different end goals.

Secret of Mana's art is better for Secret of Mana because the game is a fantasy epic that wants to pull you into the fantastic naturalistic world with detailed spritework.
Undertale's art is better for Undertale because it's a heavily ironic and comedic game where the simplistic art style is heavily incorporated into the comedy. The game would be MUCH WORSE if it had more detailed sprites like Secret of Mana.