My mind is blown by the idea people would go to centre Parcs with NO kids?!?! I actually cannot fathom why this would be...
Because we like it, and you can have a great adults only trip there?
It isn't too far away for us so we can make the most of it (I've done day trips there), the spa is fabulous so we always have a few hours and a meal there, we have a laugh with activities like segways, archery in the forest (rather than just plain targets), golf, quad biking, and even the pool (because you're never too old for a water slide), then nip back for lunch followed by playing badminton/squash/tennis or cycling. People can dip in and out as they like with ease, or do a different activity like a class instead.
We go outside of the school holidays so it isn't packed, there aren't many children other than younger ones who aren't in school, is much cheaper, and is a nice way to get friends together. We take it in turns to cook so no one is overburdened and try to make sure we get either one of the higher end lodges with a hot tub/pool table, or at minimum one with a little sauna room to relax.
It's a completely different type of break to getting a cottage, going for a city break, or a pool holiday, or a festival, or skiing all of which we also do. Yes we can probably do all the same activities nearer to home for cheaper and use the money for one of those instead, but that means travelling between the activities and you don't get the fun of staying together as a group, plus it ends up being harder to get everyone together rather than having a dedicated weekend away.
I have no doubt we'll continue to go when we have DCs, but again, it would be a completely different type of trip. Each to their own.