There's a number of factors at play. I can't speak for Android, but under iOS UIKit is extremely deep and rich in its offerings while also being (for most apps)

There's a number of factors at play. I can't speak for Android, but under iOS UIKit is extremely deep and rich in its offerings while also being (for most apps) highly efficient. It's been honed not only nearly a decade of mobile development, but also by the decades poured into its desktop ancestor AppKit. This is signified by how the vast majority of third-party iOS libraries are sugary wrappers over UIKit, and it's ridiculously hard to replace with anywhere near the same level of coverage, quality, and efficiency. Many have tried, but few have succeeded, and even "successes" are often fraught with compromise. So compared to the web, iOS is a totally different beast. It's very "batteries-included" – where installing a huge pile of third-party dependencies is the norm for front end web development, it's the exception on iOS. One can very easily craft a solid, full-featured, highly polished production-ready app with UIKit alone without too much issue, so demand for alternatives is quite low.