Why declare interface property as an abstract?

Question

abstract class Position
    {
        public abstract string Title { get; }
    }

    class Manager : Position
    {
        public override string Title
        {
            get
            {
                return "Manager";
            }
        }
    }
static class Factory
    {
        public static Position Get(int id)
        {
            switch (id)
            {
              case n:
                     return new ....
            }
        }
    }

Can we not just declare the property not abstract? Why it has be overridden when implemented?