Because chromatic aberration is only partly correctable, and correcting it usually costs image quality.

Software can often reduce lateral chromatic aberration by shifting or scaling color channels, similar to distortion correction. But those shifts are rarely exact whole-pixel moves, so the image must be resampled. Resampling softens detail, and sharpening afterward can increase noise or create halos.

More importantly, chromatic aberration is not just simple color-channel misalignment. In real lenses, each color channel can also be blurred differently. Once detail is blurred at capture, software cannot fully restore it. So while purple/green fringes may be reduced, the original sharpness and color fidelity are not perfectly recovered.

This is the same general reason photographers still care about other optical flaws even when software can help: the best image is the one that reaches the sensor with the fewest defects. Post-processing is useful, but it is usually a compromise, not a perfect substitute for better lens performance.