I'd like to mention that most palaces from the Joseon area had been destroyed over the years. Gyeongbokgung Palace, the main palace of the Joseon Dynasty and the largest one in size, (and most likely the one that you had visited) was a place that was originally filled with various types of buildings. This is a modern recreation of a map called the Bokgyeol-dohyeong(복궐도형,北闕圖形), and it is basically a blueprint of the palace and was used during its re-construction in the 19th century. (Gyeongbokgung Palace has burnt down the Japanese invasion and wasn't re-constructed due to high costs)

If we compare that to this modern map above that area, we can clearly see that especially the right middle part of the palace area has been gone or replaced by some other building. If you go that right middle part of the palace area right now, the stuff you'd only see are open grass areas that look more like a park than a palace.
(Why are the buildings gone now? The Japanese destroyed them during their colonial period. Most buildings were re-built after that.)

This photo of Joseon (estimated to have been taken around 1880) isn't showing off a random Korean neighborhood, it's showing off the Gyeongbokgung Palace, which is clearly extremely packed.

The main large area you'd see when entering the Gyeonbokgung Palace really seems to be a large open field leading to the throne. That place was left open because it needs to be an open area for ceremonies and rituals. For example, this is the Gunjeongjeon-jinha-do(근정전진하도,勤政殿陳賀圖) and it's s drawing that depicts the moment when King Gojong's foster mother became 80 years old. We can clearly see that the center place is filled with people and the necessary equipment for the ceremony. If you go to that place now, it definitely would look like a really large batch of land (with a bunch of tourists).

Joseon's palaces were really really compact places. Basically, everyone who's involved with the palace lived and worked inside the palace area. Various facilities for the king and his family members were necessary, and other facilities such as Confucianist education buildings were absolutely necessary. Unlike, for example, the Palace of Versaille is a personal residence and party room for the King, intended to be a place of beauty and elegance, Korean palaces weren't made for the royal's maximum pleasure. (since that would go against the Confucianist ideals and the courtiers wouldn't be happy)

Well, then why not just jam all of this important royal stuff into a single (or a few) giant building like the ones in Europe? Korean architecture does not involve that much stone. Famous palaces like Buckingham, Versaille, or Schönbrunn are all made out of various types of stones. With stones, people can easily create large structures with multiple floors. Korean (and also East Asia in general) did not use stones as a way to build-up giant structures. The first stone structures in this western-style were only introduced to Korea in the late 19th century. Wooden structures back then couldn't simply withstand a giant size like the ones in Europe. Additionally, wooden structures with multiple stories wouldn't be able to reduce the noise between floors. Also, Korea has this traditional heating system known as Ondol(온돌). Ondol is a system where they heat up the floor, instead of the western furnace style which really only needs a chimney. They get the heat through the smoke that was created while cooking food/rice. If Korean palaces were a giant singular structure, it would be hard to well distribute the heat on a giant structure like that, and people on the second floor wouldn't be able to receive heat at all.

Another reason could simply be 'because China didn't do that'. Korean castles, especially the main Gyeongbokgung Palace, were based on Chinese design. The Chinese book Rites of Zhou mentions a thing called 五門三朝, or the "five doors and three sections".

'The King at the Palace". a book by the National Palace Museum of Korea says this:

"The royal palaces of the Joseon dynasty were the embodiments of traditional Confusion systems transmitted from ancient China and the products of accumulated experience and customs of the palace architecture passed down from previous kings. The Rites of Zhou, the Boom of Ceremonials and Rites, and the Book of Rites give a brief description of the spatial division and function on the royal palace, introducing concepts such as "front courtyard and back bedroom" "five doors and three sections" and "three doors and three sections".

The spatial structure of the royal palaces was heavily influenced by these Chinese ideals, and if China didn't do them, there were no big reasons to make a big move and create a totally different layout (such as a single building with everything) of the palace.

TLDR: Joseon's Palaces weren't places like European ones where it's only really used for the protection and enjoyment of the royal family. It was a place of politics and everything important to run a nation. Therefore you'll need a bunch of workspaces. And these workspaces cannot be in a single building because 1) Korean architecture cannot create an ideal palace like those in Europe and 2) people need space for rituals and stuff. Also, why make a super unique design if China ain't doing it?