They're not exactly synonymous. Example: Something can be simple (simple to understand or with very few steps to achieve it), but still hard when you do this simple process. Something can be easy to do, but very complex, with a lot of steps to do. For this translation, to avoid confusion, it's better in my opinion to keep it simple, easy=facil and simple=sencillo. (The word "simple" also exist in Spanish, but I don't know if it's a synonym of sencillo)
The fact something is simple, makes it often easy, and it's the reason why we find the meanings very close, but if 2 distincts words exist, I think there's a reason.
At some point, DL has to make a distinction between similar words. Otherwise, we would use one of the words all the time and never learn the other. (DL is all about learning vocabulary after all)
DL could be more helpful, however, by creating sentences and context that make the differences more distinct.