the desire not to make a complete arse of oneself acts as a great incentive to get a bit fitter.

Same reason.

I do events (sportives and enduros) to give myself a target to train for, to go away for the weekend and ride in nice new part of the country, for the sort of on-the-spot camaraderie you sometimes get and the novelty of riding with loads of people rather than on my own.
While I’m never going to ‘win’ anything I do keep an eye on my time and like to make sure I’m at least in the top third or quarter!