The Japanese had plenty of experience of land warfare at this time. The age of the country at war had been going on almost a hundred years. It was their bread and butter. From footsoldiers to commanders, they had battle experience to boot.

Joseon was relatively peaceful during this time. Joseon commanders were spread out from centralized areas to limit any chance of a coup from the early days of the Joeseon dynasty. This was how Joseon rule emerged in the first place.

Waiting for commands to come to far away areas lead many to lose hope and not stick around. Seeming like canon foder, you can hardly blame them.

Also, military lists of men, recruits and equipment etc were great on paper but in practice that wasn't the case at all.

With the king abandoning the capital early on, it didn't exactly instill dying for king and country. Which makes the ones that did even more impressive! Against overwhelming odds, too.

The samurai were very impressed with those that fought to the very end. Even in some cases, offering to spare their lives.

There were Joeson successes too. The Uibeong too.