If a single sniper could terrorize and demoralize an entire squad, and rack up triple-digit kill scores, a whole army filled with them must be a nightmare to face for a soldier. The chance of being hit by the first shot from an unseen enemy 400m away would dramatically increase.
Sure, it would increase the cost of the gear for each individual soldier, but a tube with two lenses and a crosshair can't be that expensive compared to the intricate rifles and rest of the gear.
And sure, what really made the snipers so deadly might have been their skill with the weapon, but it feels like every rifleman would greatly benefit from magnified optics in a long range fight.
There are many, many reasons:
Because a rifle is only as good as the soldier who wields it. Training snipers takes a long time and is very expensive. War isn't a video game.
Scopes mean nothing if they aren't calibrated and cared for.
Sniper rifles, especially those at the time, took extreme care to keep in operating condition.
Infantry firefights weren't particularly deadly in WWI. Artillery was the real killer.
After some digging, these three articles seems to answer it quite well:
I'll spare you the reading and try to sum it up:
Before and during ww1, the weapon manufacturers went for longer barrels and bigger bullets, as that increased accuracy, stoppingpower and range. That's basically what they knew how to do in order to improve the guns designs. Thus rifles eventually came to the point where the soldier were supposed to be able to hit a torso-sized target at 600m.
Problem was, most engagements started at 300m or less, due to the troops seldom being able to spot enemies at longer ranges (because of terrain, weather conditions and camouflage). At 300m and less, it seems that ironsights were sufficient to hit accurately. And even if they spotted unaware enemies at say 600m range, the squad might hit with the first shots if they had long-range rifles, but when the enemy was aware that someone was shooting at them from long range, they could just sprint between cover and close distance. Hitting a sprinting target at this distance was really hard, so even in open terrain, they could sprint and change directions at random to throw off the enemy aim.
What did work at ranges of 600m was machineguns and mortars that could blanket the area with fire. So basically by 1945 the german squads would typically carry something like 2 mg42/34 machineguns, have 4 ammo bearers, 1 semi-auto long range rifle and the rest stg44 assault rifles or mp44 submachineguns.
And as ww2 progressed, and in later wars, armies turned more to automatic assault rifles as they were better suited for the 1-300m engagements than the semi-auto and bolt-action rifles, and for longer distances than that, machineguns and artillery support was used. After all, artillery couldn't be used if the enemy decided to engage close-range with the troops, for fear of hitting their own, so it made sense to focus more on close-range weapons for the squads.