I suppose this isn't very personal and may be pretentious, but zoomers take for granted what Undertale achieved artistically.
Before Undertale there were very few games that could really be considered good art in the medium. There were film games, fun games, and extremely niche, limited titles that were often hard to even acquire. Undertale was the first real game to exploit the potential of the medium to a substantial degree and achieve something that could never be done in any other.
Ten years later and there are many GOOD games that fit that description, let alone mediocre ones, but before Undertale you could count them all on one hand. As someone who was starving for something like that since childhood, I will always consider Undertale to be a personal favor from Toby Fox that I won't forget.