Because SizeF has a default TypeConverter defined:

[Serializable, StructLayout(LayoutKind.Sequential), ComVisible(true), TypeConverter(typeof(SizeFConverter))]
public struct SizeF
{
   ...
}

While PointF has not:

[Serializable, StructLayout(LayoutKind.Sequential), ComVisible(true)]
public struct PointF
{
    ....
}