What a person needs,and what a person chooses to use are not dependent on one another at all.
I personally want at least 15x for 1000 yard target shooting.Some 1000 yard target shooters do use 40x scopes,but 20x to 25x is also quite common.Then again these are dedicated competition guns that aren't normally used for hunting.
The military snipers are normally issued either 10x or 3x12 scopes on their standard sniper rifles,which are considered effective to at least 800 yards.Even the 50 cal barret sniper rifle is issued with a 4.5x14 scope,and it has been successfully used out to over 2000 yards.Either these magnifications are sufficient,or the military snipers are very poorly equipped.Then again based on their very successful record,that doesn't appear to be the case.
Carlos Hathcock for one,was very successful with a 10x scope.

You may choose to use a 6.5x20 scope for big game hunting,but it isn't necessary.If you decide to use one it is because you want to,not because you need to.