When it comes to Spongebob Squarepants it's because it matches my own wackiness and mild insanity. I grew up on junk food, sugary drinks and had a pool of untapped energy wasted on watching shows like Oggy and the Cockroaches, as well as Ren & Stimpy. Those shows were so zany that it required all my energy to focus on what was going on, as well as laugh so hard at the brainless humour that went on. Even as an adult I find the kind of humour Squarepants offers is brilliant, especially because it tells jokes that I've always wanted to see. My all-time favourite gag was in the movie, where Dennis the antagonist takes off his sunglasses to reveal another pair, and I've never seen that done before.
So I guess you can see why children love that character - the plots are easy to understand, the characters are goofy and they're so dynamic it requires an imaginative mind to be able to follow the exploits in Bikini Bottom. There's few shows like SB SP nowadays because of role models, when Oggy and the Cockroaches were role models for would-be psychopaths.
Finally I think the term 'obsessed' is overdoing it. There have been cases where people worshipped characters but that time I believe has passed. To fully understand why they're fanatical about these characters though, you have to place yourself in their shoes with their imagination and wildest fantasies - what we think is delusional for adults is just a normal day for an energetic and imaginative child.