When I was married and where I was from, we did not include response cards in invitations. Weddings were cake, punch, coffee, nuts, mints, maybe small sandwiches and a reply really wasn't needed. You used an estimate of 75% attendance and you were really close. There was always extra made and I was never at a reception where there wasn't enough food.
When people first started serving meals in our area, they were not sit down affairs but a bbq dinner or chicken dinner and the same rules applied with the percentage used.
When people started having sit down dinners or an entree where a caterer charged per head, then it seems response cards started showing up. When you were paying per head or per plate, you wanted a more accurate count.
Most invitations we get now do have response cards. One or two have had the website response option. We have gotten a few over the past years where there wasn't one included and it was generally for a reception at the church or at home and no response was required as the family was making the food.
So, if there is a response card I will fill it out and turn it in. If there isn't one, I don't respond and have never been contacted for a rsvp.