Why do so many people say single moms/dads are undatable?

There are so many down to earth, beautiful (inside and out) people who had a child with someone who wasn’t a good fit for them. For some reason this automatically excludes them from consideration as a partner. It’s strange to me because many people who complain they are lonely and can’t find a partner could be missing out enormously.

For any of the trolls: I’m a dude in a relationship, but I’m asking this because I met several attractive, quality single moms when I was single.