While this bears no relation to physics, a basic thought experiment will answer your question.  Take any sport with which you have familiarity; swimming is excellent for the purpose as the minimal gear shows the physique clearly.  All participants at a high level are in excellent physical shape.  Shoulder widths differ as does the slope of the shoulders.  Some are long-waisted, some short-waisted.  Some have muscular thighs and others have longer and leaner limbs.  For those with identical chest measurements, some will be quite thick from front to back, and some will be narrow, but wide.  Hip width and circumference will also vary for people with the same chest or waist measurement, and so on.

If you are a manufacturer, which of these athletes do you design your suits for?  If you fit one of them well, you have guaranteed that the others will not get a good fit without alteration.  If you decide to take the middle ground on all of the various fit parameters, how many people in the population actually have that exact physique without some significant deviation somewhere?

Overlaid on this is the actual construction phase where a quality bespoke tailor cuts the garment differently and then builds in a shape that fits even your athletic ideal physique even better.

That said, it is still difficult to argue for bespoke as a "need"...