Why can't I adjust my iPhone 16 flashlight to a narrow beam by swiping left to right?

The iPhone 16 does not have that feature. The only iPhones that let you adjust the beam width are certain Pro and Pro Max models that have Adaptive True Tone Flash.

The flashlight beam width control relies on manual control of the multi-LED flash array that supports Adaptive True Tone Flash. No LED array, no ability to control beam width.

See this link and pay attention to the section about changing the size of your torch beam with Dynamic Island in iOS 18 --> Turn the torch on or off on your iPhone or iPad Pro - Apple Support (IN)

The Support article seems to assume that the Dynamic Island is only found on Pro and Pro Max models.

I believe that the plain iPhone 16 also has the Dynamic Island, so the article’s implication that the presence of the Dynamic Island means that you can control the beam width may be in need of a correction.

Servant of Cats wrote:

The Support article seems to assume that the Dynamic Island is only found on Pro and Pro Max models.

I believe that the plain iPhone 16 also has the Dynamic Island, so the article’s implication that the presence of the Dynamic Island means that you can control the beam width may be in need of a correction.

Yes, I agree. I couldn't find anything which talked to the models that support what was being asked. And I have a 16 Pro Max, not a 16, so I wasn't aware it requires adaptive true tone flash to do this.