Because the aperture works best as part of the optical design of each lens. In most lenses, the diaphragm sits between lens elements, not simply at the rear. Its position affects how the lens forms the image, so moving it to the camera body would greatly restrict lens design and optical performance.

A body-mounted diaphragm would be especially problematic for wide-angle designs and for cameras where sensor size and flange distance already limit rear-lens geometry. Many lenses also have optical elements behind the diaphragm, so a single aperture in the body would not substitute for the lens’s own aperture placement.

In short, the aperture is not just a light gate; it is part of the lens’s optical formula. Keeping it inside the lens gives designers flexibility to optimize image quality and make a wide variety of lens types possible.

Related point: shutters can also benefit from being placed in the lens (leaf shutters), because they operate where the light cone is narrowest and can achieve very fast flash sync. But unlike the aperture, a shutter does not shape image quality in the same way, so putting one in every lens is usually harder to justify due to cost and other tradeoffs.