If someone cannot take the time to write a simple thank you note, perhaps the giver does not have the time to go to the store to buy the gift or take out a pen to write a check. There are plenty of inexpensive TY notes or buy a pack of pretty paper(100 to the pack) and make your own. It does not have to cost much. That’s a poor argument (pun intended).
It really is NOT enough to say an obligatory thank you at the wedding especially when you have no idea what the gift is, until you open it. Young people have time for every naarishkeit these days. Its not too much to ask them to show that they are makir tov. If you can not show appreciation to people for doing one nice thing your you, how can you show it to HShem who is CONSTANTLY doing for you?