It also would not make any sense that if GG could throw 507 "on a line drive" he would do it. Why would he choose to throw a distance line?
distance contests are different from regular throwing. kallstrom is one of the few that will throw normal line drives in a distance contest. the thing is, people aren't trying to throw 500' in distance contests, they are trying to throw 700'. each round also only gets 5 throws and it's within a time limit. the other thing is, huge line drives and huge distance lines don't happen all that often. it might take someone 30 tries to break 600' once, and often that once doesn't happen during their 5 tries.
yah, a lot of guys would probably do better if they threw one safety, but most are going for the best chance of hitting the big one (and there's no guarantee that their 1 safety shot is going to go far enough to matter).
conditions play a big part in distance contests. you find that when conditions are good, the overall distances are better in addition to the winning distances. when conditions aren't ideal you see much lower distances winning contests. from 1987 to 2001 the distance record grew from 612' to 712'. on one day in 2003 4 throws broke 712' (all with plastic that was available in 2001). that day was an ideal day. the difference between a 600' throw and a 650' throw is often just luck. a good or bad gust/swirl late in the flight can have a severe affect on the flight of the disc.
basically, in distance contests people go for it all because they know everyone else is too. also, while 500' is doable on a line drive, 700' isn't.
at 2004 worlds distance contest kallstrom's 5 drives in the finals all broke 520' and all were line drives. sandstrom won it with 631' and that was his only drive that broke 450' out of his 5 attempts. MDR won the advanced men @528'.