I think sportives are a way of turning a solo/small activity in to a big group one without the rider having to do anything like joining a group. A lot of the draw is probably the achievement factor, it's one thing to map our your own route and do it, it's another to force yourself to do someone else's. I think that's the big draw for my wife, you can't just select easy routes and avoid all the hills with a sportive where as you can with your own ride.
On the challenge front it's also a much more concrete thing to aim for. "I'm doing this sportive on this day so need to be ready" is a much better target than "Saturday I'll nip out and do this route" for people who are at the beginning end of cycling that challenge is a really good reason to get better.
There's also the element of being able to see where you are next to other riders without having to do anything serious like starting to race. That is a *thing* for some people. I kind of like to know where I am ability wise just so I can work out what my possibilities are compared to other riders, Strava can do that, but it doesn't really say how far the other riders had gone, or what the weather conditions were that day.
Then there's the odd really special thing. The only sportive that's really hooked me is Ride London and that's really because of the uniqueness of riding those roads closed.
And I agree on the rubbish, I have an 'empty wrappers' pocket that gets filled up and emptied when I stop near a bin. Don't find it that difficult, not sure why others do.