Why don’t DSLR cameras use a protective curtain to cover the sensor area during lens changes?

Because it would add cost and complexity while doing very little to solve the real dust problem. Dust is not only airborne during lens changes: there is already dust inside camera bodies and lenses, including on the rear of the lens, and once the cover opens that dust can still reach the sensor area. A moving curtain would also need space, could fail at the wrong moment, and its own mechanical wear could create more particles inside the camera. In other words, it’s another mechanism to design, fit, and service, but with limited benefit. Similar protective curtains have existed on some systems for other purposes, but they are not very effective as dust barriers. So manufacturers generally prefer simpler designs and rely on normal sensor-cleaning methods, careful lens changes, and dust-reduction features instead.