I enjoy date math. Thomas explained the answer above.
If you add an integer to a datetime data type, it adds that number of days and returns the correct datetime. This meets the standard of date + interval = new date.
Not so with datetime2. It gives an operand type clash error.
The one that really gets me is the date data type. It gives the same error.
Microsoft did a lot of things right with the datetime type, but missed the boat with the newer ones. I don't know why they made the decisions they did.
DATEADD works with both, but not straight addition and subtraction.