You likely find it stressful since they are completely and totally ignoring you and not even trying to be polite or nice about it. Its one thing to be part of a conversation and not really contribute to it (but are at least paying attention and trying to absorb the information) and another to be "part" of a conversation and completely oblivious to said conversation.

This has happened to me a lot in recent years and I find it stressful as well. Probably the biggest problem I have with it is I don't understand the purpose of going out somewhere with people to not participate at all. Why not just save everyone time/trouble and not go in the first place?