In addition to o.m.'s answer, it is worth commenting that it was not as if commanders refused to try new solutions to avoid the horrifying loses and break the stalemate:

• Use of artillery was progressively increased to provide more and more cover.

• When it was discovered than an excessive use of artillery helped the defenders by providing advance warning, creeping barrages were developed.

• Gas and flamethrowers, tanks and stormtroopers were devised and used.

And, when introduced, those solution did work! But usually they were used first in small, test attacks¹, which gave the enemy enough time to recognize the threat and adapt their defenses.

Add to this the natural tendency towards confirmation bias (or simply optimism). The front was not totally static, small advances were made, and confirmation bias made commanders believe:

• that their operations were more successful than they really were, by overestimating enemy casualties,

• that, because of the above, just "another push" would definitely "break the enemy" for good,

• that anything positive from those operations was the confirmation of their good leadership/improved strategy², while anything negative was due to other factors: "thanks to the barrage of artillery that I designed we advanced a kilometer, but we were stalled short of breaking through because the men could not move fast enough"(disregarding -or even not having been told- that the men could not move fast enough because the barrage had made the terrain impracticable, and that despite the artillery there was still enemy resistence slowing the advance).

And yes, you could qualify those points as "bad leadership", but it is not as simple as "they simply did not care".

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¹And of course, we know now what worked and how it had to be used to be effective, but at the moment sending hundreds of thousands of soldiers to a big offensive on the basis of new, untested tactics and equipment was the kind of risky behavior that could lead to a massive amount of casualties.

²The generals wanted to have a solution to the stalemate/massive casualties; when they thought they had found one they were reluctant to leave it because it would leave them with no alternative to the slaughter...

Not until the time of the tank.

"Repeating" rifles of the late 1800s brought about the rise of trench warfare. (Ditto for other "repeating weapons such as machine guns and Gatling guns.) That's because a defender with such a weapon in a static position had the advantage over an attacker who had to run and fire at the same time. Trenches, and other fortifications such as barbed wire that protected defenders amplified these advantages. So for the first two years of the war, the British and French allies couldn't do much against the Germans in trenches, even with a 5-to 4- numerical advantage. At that time, they could not be accused bad leaderships.

The arrival of the tank in 1916 changed the picture. This was a weapon that could "collapse" a trench, nullifying the defenders' advantages. Initially, they were used as infantry support: The "Blitzkrieg" tactics of World War II were 20 years in the future. Nevertheless, a mass of several hundred tanks could be used to break a trench line manned by infantry, as was the case at the battle of Cambrai (1917).

After Cambrai, the Allies could be accused of "bad leadership" for not taking better advantage of the tanks that they had a virtual monopoly over. Ironically, it was the Germans that learned the lesson and who made "tanks" the focus of World War II land operations.

Hindsight is cheap, and often misleading.

• Officers liked to believe in dash, elan, and maneuver warfare in general. It gave them the hope that by being better, braver, more daring than the enemy they could make a difference. Read The Charge of the Light Brigade.
• Generals liked to believe in strategy, maneuver, and the short victorious war. It allowed them to promise an affordable victory to their kings and presidents.
• And maneuver warfare and a short war might have been possible. If a French corps had marched faster here, if a German advance had taken a fortress there by coup-de-main, if a pair of Russian armies had been better coordinated and better supplied, it might all have ended before the stalemate began. The answers to those "terrible ifs" don't quite belong on History.SE, realizing that the questions exist does.

If any of those "ifs" had happened, if the Russians had taken much of East Prussia, if the Germans had raced through Belgium to find the French slightly more off balance, it might have become another 1870/71.

Much of the World War One slaughter was more due to a lack of imagination, coupled with a distrust (in European armies) for non-comissioned men.

Read a good book on the Vimy Ridge assault by the Canadian Corps, under Arthur Currie. Two fundamental changes were made in preparation.

Information

Rather than issuing the timetable to every commissioned officer in the Canadian Corps, Currie had the timetables and objectives printed and issued to every rank down to lance corporal. This ensured that units could continue on the proper schedule even in the event of suffering casualties to all officers. This was important because the intent of the walking barrage was so that assaulting troops would be in the German trenches less than 60 seconds, ideally 30 seconds, after the barrage lifted.

Individual gun calibration

As guns fire a long barrage the barrels heat up, and their range drops due to the increased windage. Before Vimy, generals on both sides calibrated guns by their age, on the premise that anything more precise was pointless. This led to barrages walking backwards, generating friendly fire casualties and preventing assaulting troops from getting in to the enemy trenches soon enough after the barrage lifted.

In the 4 months preceding Vimy, Currie had every single gun individually calibrated, every 5 shells. During the attack itself, this allowed the Canadian troops to stay within 50 yards of the barrage, easily getting into most of the German trenches within 60 or even 30 seconds.

The result was a successful capture of Vimy Ridge with only 1/4 of the casualties sustained during the unsuccessful French attempt.