Life or death
In the time of the Samurai time was a matter of life and death. The swordsman had to be so well trained and skilled, because any hesitation or delay on the battlefield would mean death. This is also the origin of mushin, or no mind (無心). Being able to react faster than the opponent would save the live of the Samurai. Over 800 years of evolution, the swordsmen with the best techniques were the ones that survived.