Einstein is such a powerful force in modern culture that his name is slapped on many quotations that he is not, in fact, responsible for.
The origin of the erroneous quotation “everything should be made as simple as possible, but not simpler,” is discussed in some detail in this article, which includes references, from quoteinvestigator.com. It notes that Alice Calaprice, the editor of “The Ultimate Quotable Einstein”, believes that Sessions remark appeared in the New York Times in an interview (“How a ‘Difficult’ Composer Gets That Way”) published on January 8, 1950. There Sessions attributes the phrase to Einstein, but one can read him as stating it as paraphrase, not quotation. “I also remember a remark of Albert Einstein,” says Sessions, “which certainly applies to music. He said, in effect, that everything should be as simple as it can be but not simpler!”
Einstein did say “It can scarcely be denied that the supreme goal of all theory is to make the irreducible basic elements as simple and as few as possible without having to surrender the adequate representation of a single datum of experience.” But this is a statement only about theoretical science, not about literally everything!
What Einstein is really articulating here is the basic tenet of theoretical physics: find the most efficient equations and concepts as possible that can capture the results of all known experiments in the field of study. Einstein is not giving this as general advice for life, and certainly not for writing books. His remarks have a certain relation to Occam’s razor, attributed to William of Ockham, a 14th-century English philosopher and theologian. His “razor” is frequently cited as Entia non sunt multiplicanda praeter necessitatem, which translates as “Entities must not be multiplied beyond necessity”, although Occam apparently never used these exact words. The point is not that simpler is better in general, but that the simpler of two equivalent explanations is the better of the two.
Important note: Occam’s razor does not mean that given two different explanations, the simpler one is likely to be true. History has shown that this is often false, not only in particle physics but also in other areas of science. Unfortunately, the razor is often misinterpreted in this way, even by scientists!
