My hypothesis: Coffee shops (Starbucks and knockoffs) are very very popular in Seoul, but are relatively expensive compared to other cities. Koreans are just being economical about getting the experience.
Why are they so popular? There’s always been a coffeehouse culture in Korea, part of the “bang” culture that carves out temporary private space in dense urban environments. Historically the quality was not so great, often instant coffee, and the rooms were darker and cozier, but Starbucks introduced high quality roasts and modern glassy interiors. The new shops became a status symbol, where young affluent people could meet and be seen, then quickly became the norm as competitors rapidly sprang up overnight (retail trends move extremely quickly in Seoul).
Meeting up with a friend or a date usually starts or ends at a coffee shop, often as a way to reciprocate after one person pays for a meal. You get used to spending a lot of time in them, until you realize how much money you’ve spent…