I live in developing countries. It's true that smartphones are the only computer people have or need, which is a problem.
Mobile devices are still consumption devices. It's hard to create complex, multi-layer structures on phones: the screen is too small, the processor is too underpowered... take your pick. But creating a good SaaS or a good UX on a phone is darn tough.
Here comes speculation:
Websites/services in developing countries are painfully bad.
This is due to a number of factors outside technology, but it's also true that universities here graduate people who never grew up with big screens and unwalled gardens, never grew up creating rather than consuming, and don't have a sense of what it's like to go from blank screen to working prototype to polished platform.
Neither do most people in developed countries.
But at least computers are widely available in developed countries so that the x% of kids who have an affinity for these things end up getting started early.
Someday mobile devices will match and surpass desktops and laptops. But that day is not today, and meanwhile developing countries are years behind not only in physical infrastructure, but in online infrastructure as well.