It does not matter where love and patience and nuturing come from. Children need stability. Grandparents probably live in the same house for a long time. They may even have the same car, furniture, and pets for years. Even if grandchildren don't visit very often, familiarity is comforting.
Older people tend to be more routine-oriented, and children like agendas.
They feel safe knowing what will happen at Grandma and Grandpa's house if it's basically the same every time they are there. Each of us is a product of our experience and the combined influences of people we knew when we were growing up. Never miss an opportunity to have an impact on a child's life. When a child pays you the ultimate compliment by asking you to color with them or play outside with them, jump at the chance. Everything else can wait.