In America as well as many other nations, family size has gotten appreciably smaller over the years, for a whole slew of reasons, which would include such things as: less marital commitments, marriages at a later age, first child being born at a later age, birth control, a commitment to having just one child, and so forth. It obviously isn't an accident that family sizes have gotten smaller, it is the preference of many couples and the trend does not show any signs of slowing down, as children are both a big responsibility as well as expensive to have.
While the above does go a long ways towards explaining the rise of one-child families, what often isn't looked at so carefully is the fact that children without siblings are not growing up under the same circumstances as children that have siblings. That is to say, children will deal with their parents on a level that is different than the level that children deal with their siblings, and when an only child has no siblings, they are missing the positive aspects and the common ground of having a sibling. While there are substitutes for this lack of siblings in the sense of having cousins or friends or things of that sort, these substitutes cannot readily stand in for someone that lives in the same household as yourself and therefore experiences the same sorts of things that you daily experience and hence are able to confide in, one to another.
Parents think that their children love them, and often they do, but that love is simply not the same love as sibling love, because children do not think the same thoughts, or have the same priorities as their parents, because they are children and in the process of growing up, which has its own rules and complications. This means that while a child can confide in their parents for all sorts of things, children with siblings, prefer to confide with those that they flock with, because there is a simpatico relationship between the two. No doubt, some siblings don't get along, at least from time-to-time, but most siblings believe that the advantages of having a brother or sister far outweighs the sharing or selfishness or unfairness or reduction in privacy that they may perceive also from having a fellow sibling in the household.
None of the above, means that it is wrong for parent to just have one and be done with it, far from it, it is more meant to reflect that children like to hang out with other children that they relate to, and typically most children relate rather well with their own siblings because they pretty much have the same background and that makes their credibility appreciably higher when engaging with each other. So too, having siblings, means that you have backup, not only when you are growing up, but also when you get older, which is beneficial to children, but also, beneficial to parents, as some children are more caring and more responsible than others. So then, it isn't a fair expectation to believe that just one child can be it all, perhaps they can, but it might all be easier with that teamwork that comes from having more than just one.