I never said all families. But, the truth is, families are less likely to invite a solo person into their activities than are a couple adults. Often it's a security thing - you're going to be more wary and protective around your kids than you are around your adult friends/family.
I've done a lot of solo traveling. Older couples and pairs of women have realistically been the most welcoming. It's not a negative on others - heck, I don't expect anyone to "take me in" and make sure I enjoy my solo trip, let alone blame people for not doing so.