Built-in ND filters are uncommon mainly because most camera buyers don’t need them often enough to justify the added cost and complexity.

Another big issue is flexibility. ND needs vary a lot by use case, and one fixed internal filter usually won’t cover everything. Variable ND solutions are harder to make well, can be expensive, and may introduce optical drawbacks, especially at stronger settings. That makes them a poor universal substitute for external fixed ND filters.

There are also optical design tradeoffs. Adding a filter inside the camera changes the light path, can affect focus calibration/back focus, and introduces two more glass-air surfaces that can reduce light slightly and increase reflections.

Because of this, built-in ND tends to appear mostly in cameras aimed at video users or certain fixed-lens models, where a small number of fixed strengths can be genuinely useful for controlling shutter speed in bright light. For general interchangeable-lens consumer cameras, external filters remain more practical because they let photographers choose the exact strength needed without forcing every buyer to pay for a feature many won’t use.