I would say its because those "mistakes" are profitable to some who are in a position to ensure nothing shatters the illusion of intelligence with which we are endowed.
Because of temptation.
because we're stubborn and perhaps decide to face the consequences afterwards
Because our basic motivation...greed...hasn't changed at all.
mistake is a judgment after the event .... very few events are under control some of them just happen ... you can watch try but difficult . simply saying the judgement on the victor of a war is almost always fair .
It's more comfortable to point out the mistakes of others and pretend we're "cool". Also, I think, as a whole, people live their lives more in busy and reactive mode, rather than reflectively, which is key to learning from our mistakes. Then there's humility, which helps in learning from our mistakes, but is in short supply in a society where people are compelled to "sell yourselves" and have self esteem.
It is a question asked countless times and is never to be answered. Man has never learned from history and has repeated his disastrous mistakes over and over again.
There is always a new tyrant or a supposedly new idea to fight for which is nothing more than rhetorical nonsense. Wars are fought over trifles and men, women and children die needlessly at the hands of narcissistic monsters with an agenda.
A new world order is always just across the horizon . Either a Caliphate the very old but now current idea germinating is generating a genocide. The thousand year old Reich, as in Hitler's time will probably again arise. Russia again is asserting itself on the world scene through blood shed imposing by force its ideas on others. China, also is flexing its military might and will encroach on its neighbors.
It is so basic flaw in man's make up that for thousands of years we have not eliminated it from his collective psyche.