I think there is more than one reason for this. For a start, celebs live totally aspirational lives - they are rich, they are beautiful and people will do anything to please them. They command enviable power in our society. Humans by nature are attracted to that, even though these attributes in the celebrity world are usually superficial.
Also, the way celebrities are packaged by the media dehumanise them. It is easy to build them up, root for them and then tear them down in the same way you would for characters in a movie, because they are not real people to us. That gives us a temporary surge of power (think of what looking at "cellulite circles" on a model does for a woman who feels insecure about her looks). The reason why people get more into them than fictional characters is that their lives become interactive soap operas. It's happening in real time, all the time.
Escapism, consumerism, feelings of inadequacy... there are so many things working here which, as most people here have said, distract some people from their own lives which they may not be all that happy with. Our society is not set up to appreciate what we already have, but to aspire to get more and more. Those who buy into that mentality will always be obsessed with celebrities and celebrity culture.
General interest (such and such has just had a baby, how sweet) is fine and normal, as we do have interest in people in in our 'real' lives that we may not know well. But the way which we are encouraged to delve into celebs lives, with no limits to what see, causes it to be addictive (I used to watch E! and read Perez daily) gives you a short-lived boost, and is damaging to self-worth, self-esteem and general wellbeing.