A bigger question is why didn't the French learn anything from the previous battles of Crecy and Poitiers where the longbow was also used decisively?
Mainly because the French were a bunch of small Kingdoms, Fiefdoms, Baronies, and other minor aristocratic houses that had been handed down to one another for Centuries, and they arrogantly and collectively thought they were too good to be defeated by any mere English peasant “long bowman”.
Napoleon Bonaparte was able to conquer most of France because he united all of those petty principalities into one nation. Until then, nobody with any guts was left of those different Houses because so many of them were slaughtered at Crecy, Poitiers, and Agincourt. The French Revolution finished off what the English long bowmen started...
Another interesting note is Henry V’s personal attachment to the longbow.
When he was Prince of Wales and at the age of 15 or 16, he was leading one of his Father’s forces against the Welsh, who had little love for the English. Henry paused during the battle and raised the face piece of his armor to take a drink, and was promptly shot in the face with an arrow that embedded itself in his left cheek.
After the battle, the arrow was extracted, and the wound dressed with clean linen soaked in vinegar and honey, and it eventually healed, but left an enormous crater in the side of his face. That’s why there are no portraits of Henry V showing that side of his face.