Andrew.
Flat tees give no undue advantage to a lefty or a righty,
A high ball hitter or a low ball hitter,
A slicer or a hooker.
And, since architects seek to forge a tactical challenge that favors no particular game and since superintendents have to maintain a restricted area, flat tees are the product of common sense.
But, Adam Clayman's and Mike Benham's comments are also significant.
For centuries, golfers teed the ball up, up to two club lengths from the hole, and in most cases the hole was usually on an area of the green that was relatively flat.
When you combine the history of the game, with the architect's intent and a superintendent's responsibility, you get flat tees, allowing for drainage.