Maybe I'm a Northern tightass, and it's not a dig at the OP, but I've never understood why sportive participants pay so much money to ride on roads they could ride for free.
Are most of these charity events or is it the joy of riding on closed roads?
Many people new to cycling find the prospect of setting out on an 80 or 100 mile bike ride daunting.
Not so much the distance, but the irrational fear of the unknown and what might befall them along the way.
Thus the support - well sorted route, feed stations, a van if they suffer some sort of cycling calamity, is a great reassurance and makes the difference between being them brave enough to do the ride and being too wary to do it.
As an experienced touring cyclist, you would set out on such a ride without a second thought, knowing you have the ability to get round, whatever happens.
It may sound daft, but when I was a new cyclist looking at a closed road event, I didn't really grasp the benefit of closed roads.
It only dawned on me after a few close shaves.
So I reckon closed roads might be more of an attraction to an experienced rider made weary by occasional, but persistently occurring, negative interactions with motorists.