There are somewhat obvious explanations. Coding is only one subset of the skills to launch a successful software business, and not even the most important one. There's also a huge element of randomness and luck, especially for a single given project and especially for any type of free b2c social app. Also, people outside Silicon Valley bubble have access to unique social circles that can serve as community "bootstrapping", which would be especially important for a social app
Even knowing all that, there's an element of surprise when you see it happen.
I do suspect another big factor is that experienced people overthink things a bit, much like a child seems to learn a language faster than an adult but may only be more willing to make silly mistakes. Many of the best startups and indie projects would never be started by many experienced engineers because they're perceived as too simple, the market is too saturated, or simply they feel they'd look foolish for trying and failing to do something simple instead of trying and failing something complex.