This is the kind of thing I see as a family thing - if I were arranging a family get-together, and some people didn't drive (or have a car) we'd work it out, not just from the host, but a family perspective ("Who's going to pick up Mom? Stacy's coming in from university, and will need to be picked up at the train station") Same for something that had people flying in - we'd talk amongst ourselves, and work out a way to get people where they were going without undue hardship, particularly for elderly/frail members, or those who
For large events, or people who were more distant connections, or who invite themselves, I'd say it's up to them to work it out.
I can sympathize with someone who is stuck, though. If you can't drive anymore, and you aren't physically robust enough to handle public transportation, it can be very isolating to want to socialize, be invited, but have to turn it down because you can't get there. It's particularly tricky for an event where you don't know who else is invited, because you can't phone up people to ask if they've been invited to something, but you can't get there unless you can ask for a ride. [Cabs work sometimes, but they can get get very expensive for someone on a fixed income, or a student - an extra $50 to get to and from a social event is not necessarily possible, particularly on a regular basis]
My mom is currently at a family wedding, travelling with my aunt. She drives, but isn't keen on big city driving, and doesn't want to rent a car in an unfamiliar city and navigate all over the place, and really doesn't want my aunt to do the same (aunt is a scary driver in a familiar environment). So she talked to various people, they talked to others, and she's got a combination of rides which will get them to see various relatives. Many of them live in small towns which are too far apart to make cabs practical, but aren't served by buses or trains.