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There’s several reasons, but they all cumulate in “looser tolerances of moving parts.”

Some accuracy techniques are applicable in both types of actions - eg, better barrels, with better crowns and better chambers. A technique that is sometimes used on bolt guns that probably wouldn’t be ideal in a semi-auto would be to tighten the case neck reaming diameter. In both types of rifle, paying closer attention to the chambering of the barrel, improving the stock mounting, free-floating the barrel, insuring that the barrel is co-axial with the receiver and bolt all help.

Bolt guns striving for maximum accuracy often sleeve their bolts - something that just doesn’t apply to semi-autos. Likewise, lapping lugs usually won’t make any sense on a semi-auto.

The point comes down to this: In a bolt gun, if you have the time and experience (or your gunsmith does), you can create an environment whereby the bullet is pointed down the bore in line with the bore’s centerline at the point just forward of the chamber. In the bolt gun, you can carry this to the fullest extent by such techniques as sleeving the bolt, lapping the lugs, facing the bolt, facing the receiver, etc, and then using a tight chamber, or fire-forming your brass. The result will be seen downrange, especially if you’re paying attention to your loading practice, which might include setting the bullet seating depth to use up most of the leade. Semi-autos are often shooting ammo that is uniformly loaded to a specification, not custom-made for that particular rifle.

As you try to achieve tighter and tighter groups at distance (say, 1000 yards), the importance of the ammunition loading, the quality of the components, etc all become much more important than at short ranges.

As for proof: Look at the benchrest records for accuracy. There’s no semi-auto that can come close to the current benchrest records. Here’s an example:

http://accurateshooter.wordpress.com/2007/07/09/sarver-shoots-1403-group-at-1000-yards/

Spot on,,,

I put bullet seating depth near the top of the list as well

as powder type,,,

I’ve always used a “dummy” round for seating depth,,,

When it hits the rifling I give the die a 1/4 turn,,,

Some bullets just 1/8 turn,,,

IMR 4350 will fill the case leaving no airspace...

~~~WORK UP YOUR OWN LOADS~~~!...