As a single man in your 40s, the bulk of your friends are already married or in long-term relationships.
It isn’t like when you were in your teens or twenties when the lion’s share of your friends were single.
Without single friends, there isn’t much of a pool of prospective dates in your social circle.
When you are hanging out at a friend’s home for a get-together or barbecue over the weekend, all you see are your middle-aged friends who are there with their spouses, or long-term partners, with children running around.