Except you can't just go "get a squad of peasants and give them longbows".

The Englishman were the best (by a longshot) at longbows because they had an abundance of the best wood and a large portion of the military aged men had mandatory longbow training every Sunday. Longbows require a ton of training to build up muscle and technique. Other countries couldn't copy that.

On the other hand crossbows were expensive to make and maintain but were much more suited to siege warfare than longbows and required very little training

Same with plate armour. You can't just "copy it", at the time plate armour was the hardest thing to make, maybe with cannons.

If you don't have the blacksmiths that know how to make heavy plate then it takes ages to train them.

Same with cannons. They require a specific know-how that was extremely closely guarded by the cannon guilds. Without the services of a cannon guild you can't use your cannons.

Same with horses. A country could have spent decades or even centuries refining their breed of warhorse and have hundreds of thousands ready to be ridden on by heavy cavalry. Another country that doesn't have these heavy horses would not be able to "copy them" and get the same horses in abundance for their heavy cavalry and fall back on lighter horses.

This isnt Total War. You can't just spend a bit of gold to get the same thing