Why do people think that money is an inappropriate wedding gift?

I wouldn't mind giving money as a gift but I wouldn't want to be told I had to give a gift at all. I hate it when I open a wedding invitation and a 3 page list falls out; I feel as though I'm being invited for one reason only and that is to give them a gift. Just send the invitation with no list or requests of money and you'll have far happier guests, who will most likely bring along money anyhow.