[Burakki]

I'm not a psychologist either but I see two possible reasons to that, apart from what Serendipity said about desire to carry a pet's genes, which I agree about. Since hammies live only two years, it's possible that many owners want to have their hamster's offspring, hoping that they will have the same personality.
Apart from that, especially when it comes to children, people just think it's interesting and cute and funny to watch the whole process and have little fluffballs crawling around. When I was a child, our dog had puppies and I was so excited, I didn't understand why my parents would give the puppies away. Since hamsters are small animals, and many owners don't even know that they are solitary, even adults sometimes think that it's just going to be a cute and interesting process to watch.
Finally, I think some people think that they can make easy money by that. They think that if they just breed two hamsters, and sell the offspring, they will earn some money without any real expenses (I talk about irresponsible "breeders", not real ones, who know that it's a demanding and complicated process). It's not the same with cats and dogs, who grow up and run around the house, if they're not sold in time. People think that if they don't sell the hamsters, they will just be put in one cage, with no real problems.