Maybe the OP wants to be part of and experience something that is big and well known and for charity? And although full of bikes, aren't the roads closed?
3 of those 4 reasons are why I'm doing Ride London
My wife and I are runners that are new to bikes and wanted to aim for this event for all the above reasons. If, however we have to raise £2-300 between us plus the entry fee we will not do it. We've never experienced that in running.