Firstly, this post should not be necessary, but sadly some bad manners have crept in.
We have all found that despite best intentions there will be occasions, for all sorts of reasons, we have to drop out.
A lot of hard (unpaid) work will go into the organization by members to make these things happen for others. So when entrants don't turn up that might mean:
1) The event/drive is delayed;
2) If the entry list was full, others who could have made it have lost out;
3) There is concern by those who have arrived something untoward might happened to the member on the way to the event.
So if you find yourself in the situation you are not going to get to the event please let the organizers, or someone else who you know is attending, that you can't make it - we all have mobiles these days and it only takes a call/text/pm/thread post.
Yes, there will be occasions when making contact beforehand it is just not possible. But it would be good to hear afterwards that the member is OK and an apology was offered for their absence.
As I said at the start, I'm sorry for the need for this post but if a member is seen to do this all too often then don't be surprised when their name gets deleted from the entry lists.