I think you mix the meanings of simple and easy here. Simplicity is an absolute metric and describes the number of dependencies a thing has, while ease is a relative metric describing your understanding of said thing.

For example, a singleton is easy to learn and easy to use, but since every function using it adds a hidden dependency it quickly grows in complexity to the point its impossible to reason about it without forgetting something.

On the other hand, a Promise is simple as it depends on nothing but a producer and a consumer, no matter how much you compose them. Yet I've seen many experienced developers struggle to learn how to use them as they're not easy to understand at first.

This is somewhat related to meta ignorance. From my own experience I've seen a tendency in novice programmers to stick with things which are both easy to learn and use. Their projects go well initially but they grow less and less productive over time as complexity creeps in from the composition of all these easy to use things.

I've always said experience in our industry is knowing what not to use in order to stay productive in the long run.

Here's a link to Rich Hickey explaining it in depth: http://www.infoq.com/presentations/Simple-Made-Easy