Here we go with the stereotype again. Only children are not lonely children whatsoever. They have their parents, cousins, and other children as companions when they want to socialize. Otherwise, only children are happy with their own company. Being alone allows them time to create and be involved in intellectual activities such as reading.
Only children are also the most imaginative and creative children because they spend a lot of time alone. Another aftereffect of this is that they are highly independent and not as needy as those who grew up in multichild families. When children grow up with siblings, they have no concept of alone time. They are always out and about with their siblings. The thought of being alone to children in multichild families is quite terrifying. They simply cannot conceive the concept of alone time. They MUST be with SOMEONE.
Only children, on the other hand, are not uncomfortable with being alone. In fact, they relish it. If they want to socialize, they can always go to their parents, a friend, or other children. Only children also have privacy which is a foreign concept to those in multichild families. In such families, there is always a child wandering by. In my experience, in childhood, it was the ONLY CHILDREN who always had friends and/or other children visiting because the latter could have piece and quiet from their squabbling siblings. Other children seldom visited those in multichild families because it was ALWAYS noisy and full of havoc.
No, the only child is not a lonely child. In fact, he/she is blessed ad infinitum. He/she has the BEST of all worlds.