German skirmishers approach the French lines during the Battle of Sedan, in what is possibly the first ‘combat photo’! Franco-Prussian War, 1870

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Notice the soldier near the right of the first line, arms extended. He appears to be dying or at least getting hit right when the photo was taken.

There was a very good reason to remain in real “lines” at the time. Breech-loading and repeating weapons were still brand-new; in combat, the old standard of three shots a minute was still more often true than not, and even against very modern soldiers you could probably expect, at best, ten shots a second from falling block, single shot bolt, and lever weapons; less than that, accounting for maneuvering. Magazine bolt-action rifles were still almost three decades from widespread deployment, the machine-gun didn’t exist, and the Gatling gun was a joke against a modern army which could engage it’s operators at rifle range.
That meant that in order to have a decisive weight of fire on enemy formations, you needed to have sufficient density of troops firing at once. If one side maintained a modern five-meter spread and hide behind every rock and bush they could find, the other side could very simply form into a big block, run at them with bayonets fixed, and break the line, then go gallivanting around stabbing their logistical elements.
Even then, you can already see the modern mindset taking hold in these soldiers, compared to what you might have seen sixty years earlier in the Napoleonic War. Notice two distinct skirmish lines, with the reserve line maintaining firing positions. Officers on foot congregating on the ends of lines in cover rather than behind the line on horseback, to direct fire rather than steel the line against a charge. And, most importantly, notice just how many skirmishers are in this picture, in similar numbers to the line group moving up beside them! A few decades ago, skirmishers made up barely a tenth of any line unit!
And yes, there are bodies on the ground. It’s war! We don’t see bodies on the ground in modern fighting not because there are less casualties but because war is much faster and more spread out now. When this photo was taken, soldiers died in the hundred meters between two battle-lines, and a battle could only advance at the pace of men or horses walking, so naturally you had lots of bodies clumped together in a relatively small space. Nowadays, soldiers engage at three hundred plus meters with assault rifles, and artillery duels and rages at tens of kilometers. Aircraft hit targets hundreds of meters behind the lines. Armoured thrusts travel in days what would take an army weeks to cross in combat conditions a hundred years ago. So engagements are limited to bursts, on the run, over entire provinces and countries with a tenth of the forces from a hundred years ago putting out a thousand times the ordinance.
It’s very easy to look at this and see amateur or incompetent command; modern observers of the First World War often do the same thing. But you don’t realize that this was the world in the grips of massive technological, logistical and doctrinal changes, and that formations like these were cutting edge and maintained for a very good reason. Believe it or not, you are not smarter than a professional officer of the era.