Here's a little preemptive truth-telling: they don't because they know almost nobody would come to their weddings. Of course it's nice and helpful to receive an expensive, useful gift, but we can't expect all our loved ones to be able to afford or provide that. If we're setting up a venue only to receive collaborations, shouldn't we be upfront about it? So people don't think they're coming to our wedding to celebrate our love, but to give us money.
Last year my cousin got married and tried to pull this stunt, charging everyone a fee to attend his wedding. He tried to communicate it with humour, and don't get me wrong, I thought it was funny and transparent. At least it was better than silently expecting an expensive gift.
The thing is, I, for example, couldn't afford it and was too ashamed to tell him, so I ended up not going and not saying anything. So his wedding ended up being exclusively for people who had enough money, and that is sad for such an important event in one's life.
You are allowed to not like a present, but you shouldn't feel entitled to call your guest and tell them that, if they didn't know, it had to be worth a certain amount of cash beforehand. I sincerely don't know when weddings have become a full-blown cash grab.
Weddings are supposed to be a beautiful day that celebrates love, and if that comes with beautiful presents, better! Also, can't we value some things by how much time it took to make them? Is money the measure of everything?
Do you think this is normal behavior for an unwritten rule like this?