Thinking about it, I think it's the possible variety of baby that's exciting about showers. It's the unknown.
A wedding, generally, is an event celebrating one specific type of love. I don't care if you're doing a "vintage" theme, or having your stepfather officiate because he got some certificate online, or if you're getting married on the fucking moon. Majority of the time it boils down to two people standing in front of a bunch of other people and me pretending to give a shit about any of it.
At a baby shower, it's okay to talk smack about the prospective baby. It's going to keep its parents up at night, it's going to poop a lot, it's probably going to ruin lots of clothes with spit-up. These are just facts about a baby. This is why baby showers are usually designed for people to get things to deal with all this inevitable work that a baby is going to require. You can also say these things because the baby isn't there to defend itself. At a wedding you can't say anything bad because the people getting married are like...right there and they're all in the moment of being excessively happy with each other. Ugh.