I think it could be a excuse that hides other reasons people would rather not say, if it's say a pub crawl or do in a bar or similar they may have additional medical reasons (hence antibiotics or special diet) which may mean they would find it difficult to attend,
I also agree with the money aspect, having time to save up doesn't mean money is available to save, I wouldn't want to think of people having to scrimp and have to cut back of things like food or heating to come to a do of mine, especially as it seems more and more popular to have dos in posher places which can be out of the way (even weddings and hen/stag dos abroad) and involve expensive travel and hotel stays on top of other costs.
If the do is expected to be a boozy one with people getting silly drunk I can also see the reluctance of someone not drinking going if they think they are going to end up surrounded by people becoming drunken idiots - I've been to many a do and ended up having to look after people, bundle them in taxis (even go with them in a taxi while they try to remember where they live!), getting them to hospital (alcohol wise/injury), people getting into fights, police called, general drunken drama (emotional to hissy fits), lewd behaviour, all kinds of crap. Most of those night I had had been drinking too, I just know my limits and have a sense of responsibility. No, I'm not saying everyone is like that, not at all, but it does happen.