I'd say it is nostalgia. I met my wife on neo and we have a kid now so I can say that Neopets is a part of my life and it's impossible to leave/forget it xD. Sometimes I take a break but always come back.
Honestly, if I hadn't found sites like this, I don't think I'd still be on Neopets. Maybe I'd get back on every once in a while for the feels but... without the help from these sites helping me achieve my childhood dreams, it wouldn't be as fun.
10 year old me would be soooo jealous.
It's basically the only game I play and I get a lot of fun out of it despite there being some frustrating things
Also mainly because I can't have real life pets such as a cat and dog....so neo it is haha