Probably because we can't go back in time?
Oh, by the way:
..Vasily the time-traveler strolls through forest,
In distant past, in long-forgotten time,
His soul sang as victorious chorus,
And slender hands held firmly a flash drive.
His laptop filled with wonders of science,
Blueprints of A-bomb, cure from common cold,
And Mother Russia, with Vasya in alliance,
Would firmly held the crown of the world!
He brought all schemes of wonderful devices;
Tanks, planes, battleships of steel, or wood,
And also many useful advices –
Who should be shot (for sake of Greater Good!)
He went to Tsar; but Lenin would suit also;
But Stalin’s better, ‘cause he cool & smart
And German Reich’s would surely meet fiasco,
With Vasya’s help, it wouldn't get a start!
To whom exact? He simply doesn’t care
For every era the recipe is done,
Kalashnikov’s would stop the Napoleon!
And cannons make short work of Genghis Khan.
And all the Tsars, Genseks and Knyazs of Russia,
Would quickly fell in line with Vasya’s plans.
When great knowledge (stored on the laptop),
Would put ultimate power in their hands.
And Vasya could almost heard the fanfares;
From nobody he would be a superstar!
…He didn’t notice the Jurassic forest,
And Allosaurus cold unblinking eyes…
(c, Rysenok Den, with my translation)