It's not, unless you are using the term "base" to refer to the aortic root of the heart rather than the apex.
A systolic "crescendo-decrescendo murmur" is the classic description for the murmur resulting from aortic stenosis. The aortic valve is best auscultated at the right upper sternal border.
In the below diagram, the actual valve locations inside the heart are indicated by the colored ovals, but the location that you best HEAR the valve-related murmurs are indicated by the A P T M in circles for Aortic, Pulmonic, Tricuspid, and Mitral.
You hear a murmur at a different location than the valve itself because of fluid physics and acoustics. A cardiac murmur is a vibratory process occurring due to fluid mechanics; pathological murmurs are due to blood flow along abnormalities in heart structures. If you look at the course of the aorta, from the aortic valve to the carotids etc, you can see how vibration from narrowed aortic valve would easily radiate there.
Anecdotally, I've often heard it more clearly at the LUSB (left upper sternal border, which is generally the pulmonic region) than the right (RUSB), and I'm not sure why - but it was never the apex where I heard it most clearly. There must be some confusion in your sources.