I do not think many only children are spoilt but I have a much younger sister, who was home for much of her childhood as an 'only' when I and my younger sister had left home. She certainly wasn't spoilt but when she was at home as an only she didn't have to compete for, or share parental attention. When my other sister and I were home, she was eclipsed because my parents were primarily interested in us and our lives while we were there.
I didn't have the easiest of relations with her for much of our adulthood but since our parents and my younger sister died we have talked more and she explained that she saw us as favourites because when we were there she didn't get all the attention she was accustomed to and she was amazed to discover that we, her two older sisters, saw her, without rancour, as the favourite as she was very much the baby in the family.
I think DH unconsciously was the same, he never had to share his parents attention, and didn't expect to share mine. Mind you, 47 years of marriage, two DC and two DGC have solved that.