Why do people in their mid to late 30s tend to have no dating lives?

It has come to my attention that many men / women in their mid to late 30s tend to have no dating lives or its extremely hard for them to meet people. Many people claim that its this way only for women, but apparently it seems to be the same case for men too.

All of them tell me that their dating prime was in their 20s then things just stopped for them by the time they hit their mid 30s. By the time they hit late 30s, overwhelming majority of them just sit at home and complain about how hard it is to find a date.
Why do people in their mid to late 30s tend to have no dating lives?
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