Because your software should be boring and not interesting, simplicity is something you must actively work towards.

Whilst simplicity sounds easy, it often requires you to take a step back and simplify the proposed solution. A great way of doing this is by iterating over your implementation or practicing Test driven development.

It is often a forgotten, unnoticed and unappreciated skill, that will also wither without exercise. Another unappreciated skill is Glue work.

Some things to simplify