Why do modern military theorists call military geniouses stupid for making mistakes,and think war is easy?

I keep hearing modern day military theorists call Brilliant commanders in the past stupid for making fatal erros(such as Napoleon's invasion of Russia.Frederick the Great's overreliance of Heavy Infantry,and the Knights Tempklar's defeats in the Crusades),treating war like an easy game!But these military commanders had MUCH LESS access to information to the past unlike today!For example,modern military theorists keep calling the Knights Templars stupid for falling into the Arab's strategy of luring them into the desert and making the die of thirst.But the Knigthts Templars had EXTRMELY LIMITED knowledge of the Middle Easter terrain and warfare!Despite lack of information of the desert and Arab warfare,Arab generals and warriors still described the Knights Templars as cautious commanders in war who used brilliant military tactics and strategies!If the Arabs called Templars military geniuses,how come modern military theorists keep calling them stupid?These military theorists keep calling Frederic the Great a stupid commander for heavily relying on Infantry.Although his overreliance of infantry caused him some battle,it was generally extremely effective,and European generals called Frederick's Infantry tactics superbly brilliant.How come modern military theorists call him a fool?And the NUMBER 1 thing that pisses me off is that these military theorists treat war like a simple game,bragging they could easily command armies and win battles!HOW COME MILITARY THEORISTS ARE LIKE THIS?