OK, clarification..... I was speaking of a player shooting 59 or lower for 18 holes IN A COMPETITIVE ROUND (not a practice round) in a sanctioned PGA Tour event.
Look at Woods and Mickelson. Great rounds. Did just about everything right.... and still came up a few strokes short. It's just too damn hard to do. On a par-72, you'd have to be 13-under. On a par-70, you'd have to be 11-under. That probably means at least one (maybe two) eagle, nine or more birdies and NO bogeys. Conditions would have to be perfect, a guy would really have be at the peak of their game and get a bit lucky as well. It's just a LOT to ask, even more than, say, a major league pitcher throwing a perfect game.