If you'd like the less depraved answer, some of the "waifu" cards (as they are known in the community) are meta picks. The most notable example off the top of my head is Iono in the rotations it's legal, but there's others out there. Some people are willing to spend more to get a card they need for their deck.
If you'd like to lose faith in your fellow man, I encourage you to continue reading.
The driving force behind many waifu cards' high prices is their attractiveness. Many of the people that collect cards are chronically online neckbeards that scour every corner of the earth for an Evolving Skies pack and post their find to Reddit while furiously performing price calculations with their Dorito-stained fingers. Shockingly, these absolute catches can't seem to find a girlfriend in real life, so they've resorted to lusting over the only women that they can pretend will give them a modicum of attention.
Perhaps it would dishearten you further to learn that the most expensive waifu cards are those with the clearly underage girls on them. Do with this information as you will.
If you have a less jaded view of waifu collectors than me, you could say that they just like the artwork. Spend any amount of time trawling through Pokemon card collection forums, however, and you'll probably become just as cynical as me.
