Pat Burke is right on. Guys train far more now than they used to. Fit clubs, strength training and other things contribute just as much to distance as technology, if not more. When I switched from a Titleist Professional to a ProV1, I gained 8-10 yards, maybe. In the past 11 or so years, I've picked up about another 30 yards because of being custom fit for a driver and because I'm much stronger than I was 11 years ago, the same weight, but around 8% less total body fat (a decrease from like 23% to 15%). Take a guy like Bubba Watson, didn't he play fairly high level basketball? I also remember Jack Nicklaus was the same way, I seem to remember him saying he played football and all other sports as a kid, then took up golf "seriously" after he won the US Am. A friend of mine's father served in the USMC with Lee Trevino, I can tell you that requires some serious physical fitness. So, generally, I think the best golfers have always been great athletes, just not all of them are great athletes, and they all hit it a ton. 300 yard drives are not a new occurrence, Jack used to do it, Bob Jones used to do it. Now, a lot of guys do it.
People think length is the key in golf courses, its not. Make them shorter. Make the greens crazier. Make angles matter. That's what you do, not add length. Although the architects intent issue brought up earlier is major, a different ball won't fix that.
Oh, and other sports don't differentiate the rules. Tennis is played on the same court and the balls used in Pro tennis are actually "hotter" than the balls you can get at Dick's. Football is played on the same field with the same ball; times can be different. Baseball uses the same ball through the levels, bats are different. Basketball is the same from high school to NBA (regulations are slightly different from NCAA to NBA, but I suspect that is a cost issue). I can go on.