Why do couples like hanging out with couples?

[perimenofertility]

As the title says, why do couples enjoy hanging out with other couples so much?
I am single now, but when I've been in a couple I have never particularly sought to spend time with other couples so I just don't get this. I always spend time with people (single, couple, group) who are my friends.
I am in a friendship group of six people. We periodically meet all six of us for dinner. But the four who are two couples regularly arrange "couples drinks" and don't invite me and other single friend. This really irritates me and makes me sad.
Similarly, I have a good friend at work, we have lunch or drinks together often. But she regularly has dinner parties I'm not invited to because it's for couples.
So I'm wondering, what is it you couples get from hanging out together that I can't join in with?

That wasn't the point of my post - it was showing that couples are getting stereotyped on here as well.

Sorry but I disagree with the animosity towards couples on here.

Rather than animosity towards couples the problem is couples excluding single friends from gatherings for ridiculous reasons like gender balance, thinking they would feel awkward because they're single, wouldn't be able to join in conversation etc.