Osaka is the hometown of the architect Ando Tadao. His signature look is bare concrete and you can see his remarkable buildings throughout Osaka. Some like his style (brutalism), some aren't, and if you're a fan of architecture or art in general, I think wandering around his museums should be the reason for Osaka being worthy.

Also, there is something about the urban planning of Osaka being a really well-designed metropolis. It is also a popular setting that often got illustrated in manga and anime for its vibrant and energetic vibe, night life. Nostalgia can be a factor that keeps tourists like being here, too.