I must be really rude then. I nearly always do this. Not specify what people should bring, but in the fb invite (which, lets face it, is the way most of my life is organised these days) I tend to say something like "something for the table would be lovely, but if you can't do that, we will be very happy just to see you". I've even put it on the wedding invite, although, tbf, I think our wedding will be cheaper than most to attend - no formal clothes, even the bride is coming on public transport, no gifts (well, except for the food) and so on. We do know some of our friends are travelling a long way and/or are skint or too busy, but I know I enjoy making a plate of sandwiches or whatever, so maybe some of the other guests do. If not, they don't have to.
The other thing is, the more people come, the more food there is, iyswim. I'm having two parties/gathering this week alone with no idea how many people are coming. I can't cater for everyone I have invited and then chuck out 50% or whatever. Same with the wedding- we have bought a buffet for 100, but who knows if that is too much or too little.