It's probably better explained by the fact that simple problems are more likely to result in simple and smaller codebases, and the simple code and simple problems means less edge cases which means less bugs.

A lot of complexity comes with the fact that people code for the future with "well, we have to make this a platform solution in case they want to change stuff in the future, so let's add a configuration object, a datastore to pull it from, and a cache to limit database hits, with a cache eviction policy that's also configurable via ..."