Because an original mosin sniper is over $900 most of the time and an ex sniper is an EX sniper for a reason, most notable reason being that it won't group good enough to be a sniper anymore.
The 80 year old original PU scopes are also prone to leaking, scratched lenses and not holding zero any more. New copies of these scopes are probably good for about 80years before they start falling apart.
The more expensive ones MAY have a better rifle under the scope...but may not. Look at what the description of the gun is, not at the scope.
The guns are the second part of the equation and a crappy 1943 izzy Mosin with a scope is still a crappy 1943 Mosin. I've seen guns from this era that look like a beaver carved the stock and a file was used to cut the rifling. The bore was darker than the bluing and it just may have be dragged all the way from Russia behind a truck ($100 gun at best)...add a $200 scope and a $160 mount $60 worth of gunsmithing and now you have a $600-700 rifle....that may shoot a little bit better at extended ranges than it did before.
The guns that are hand selected for repro are better than. Random guns, ex snipers re fitted with a scope or the ones they couldn't sell any other way.