Why would someone do this?

Honestly, it is your fault for not having enough invitations on hand. I understand that the printer messed up your second batch- guess what? The same thing happened to me. But I still made sure there were enough invitations to go around. Your printers should have corrected the problem for free, and if they wouldn't do that, you should have taken it upon yourself to buy more invitations to be printed. It's a pain in the , I know, especially on a budget- but every guest needs an invitation, and it is simply inexcusable to expect people who can't make it to actually give you back the invitation so you can recycle it on someone else. You are blaming the wrong people here.