I think a big part of the issue is that a good portion of American shooters want to own several rifles...
I reckon that is a factor as well. Americans have much more freedom to swap their rifles quickly and easily compared to their compatriots in many EU nations. That results in many Americans owning a lot more rifles generally than the European shot and possibly swapping them more frequently.
As a case in point, here in the UK with our firearms laws, you need a 'slot' for a specific rifles of a defined calibre, say a .270 bolt action. The rifle is then purchased and the serial number linked to that specific slot. You also need a 'good reason' for each gun and only rifles with 'open conditions' or conditions specifying use on deer or other quarry can be used for that purpose.
Ultimately this means that the UK shot is only likely to have a maximum of 2 or 3 hunting rifles in deer calibres ( I have a .270 and a .44Mag lever gun for instance, with a pending slot for 9.3x74r). It is also a ballache to swap rifles. If I want to swap my current .270 for a different .270, I need to sell the current gun, notify the fuzz of the sale, then apply to my local police for a new .270 slot (at a nominal cost), get it approved with the appropriate conditions and then go buy the gun. This takes some time, varying from days to months, depending on the local force. You will also have to have secure storage space or the new gun in an approved cabinet. If the fuzz feel your new 'good reason' is adequately covered by one of your other guns, they can be difficult in granting more slots, although in fairness, there is a fair bit of leeway in most cases and they've never tuned me down.
The net result is that UK shooters tend to buy a small numer of firearms and keep them a reasonably long time. At the very least, the spontaneous purchase of something new and interesting 'just 'cus' is prevented.
As such, the purchase of a rifle in the UK is perhaps more of a big deal than in the US. You don't do it often, you'll be keeping the thing a while and you've had to wait at least a few weeks to get you slots back before you can make the purchase. As such, I reckon Europeans want to substitute 'quality for quantity', at least to some degree. After all, they might want to drop $10,000 equivalent on their firearm collection, just the same as the sportsman in Texas, but that might be spread across 2 or 3 rifles, not 10. They will probably be looking at the gun as more of an 'investment' as well, not to mention possibly saving a bit longer between purchases.
