I wouldn't call IKEA bad per se so tend to agree with you, certain products are rubbish in my opinion though, but most are quite good. One gets what one pays for I guess.
When IKEA furniture first came to Korea, Koreans were fascinated by the European-style prefabricated furniture they encountered. Koreans were the first to see prefab furniture and were fascinated by it. This is because Korean furniture at the time was not prefabricated like IKEA furniture.
The prices of IKEA furniture were cheaper than those of Korean furniture.
We tend to use IKEA a fair amount but for lounge suites we tend to opt for a more expensive, and better quality, product. Things like I bought today are decent enough though and we have many kitchen utensils, bowls, plates and stuff which seems good enough. We're not fancy people so we're generally happy with IKEA stuff.
As you say, IKEA furniture is affordable and usable. However, the current Korean furniture has also produced new Korean style furniture by imitating the style of IKEA furniture. As the quality and price competitiveness of Korean furniture improves, the popularity of IKEA furniture is relatively decreasing.