Even playing my very best golf, I never averaged 250+, nor did I need to in order to play decent golf. For me, it was simply my swing speed that kept me from consistently hitting it over 250, not a technical breakdown in my swing. Further, the vast majority of golfers don't average 250+ for whatever reason (except in Internet yards). With that said, I play regularly with a couple of guys that hit it long (265-285):
- One of them is is in his 60's and has been playing his whole life. Played D1 golf, etc.
- The other is in his forties and has only been playing 3-4 years. At 6'5", if he swings the club exactly the same as I do, he's going to hit it quite a bit further by virtue of a longer arc.
Are there things I could do to learn to swing the club faster? Maybe, but I'm not that obsessed with length. I hit it pretty straight and rarely find myself in the junk off the tee, so I have a decent chance of making par on even the longest par 4 on our course (~450 yards).
The 6'5" guy hits it about 285 but can't break 80. The dude in his sixties scares par every time out and gets it into the 60's on a regular basis. Though bashing it out there 265 helps, he'd shoot the same number, IMHO, if he magically started hitting it 245 tomorrow.
The whole question and conclusion the OP makes is really a non-sequitur. Just because one group of guys can hit a golf ball x yards doesn't mean ALL golfers with similar physical characteristics can do the same regardless of training. Kobe and MJ are the same height, yet MJ has a vertical leap that's almost a foot more than Kobe's (10", I think). Not all 6"6' guys can bang their head against the basket no matter how much training they have, just as some golfers will never hit it x yards because, well, they just can't. Just because 250 may seem to be an achievable distance by the person asking the question doesn't make it so.
One last point: in the past ten years, there really hasn't been any improvement in golf club technology that allow golfers to hit the ball farther with driver (nor will there be with any tech we currently know about). All things being equal, the only way a golfer is going to get extra yards is to improve their technique or hit a club with a longer shaft. On the technique side, I'd say that most amateurs have a physical limit that can't be overcome without turning golf into a full time job.