Personally, I think it's all greed. When you see people like Britney Spears having bridal/baby showers, it makes everyone else think they need one too. The truth is that showers were originally intended for people who honestly couldn't afford to care for their household or their children. A young couple starting out would need help if their families weren't well off financially, so the community would rally around them to help them start the next phase of their lives.
These days we expect that people who are marrying or having children are able to fend for themselves, so showers aren't necessary anymore, but it seems as though nobody wants to miss their opportunity to get free stuff. It's actually quite disgusting when you think about it.