Mostly it’s a usability and design choice, not a hard technical limit.

Cameras organize exposure in stops so shutter speed, aperture, and ISO can be traded off easily: one stop faster shutter equals one stop wider aperture or one stop higher ISO. Using standard full/half/third-stop steps makes exposure quick to set and easy to understand.

Values like 1/19s and 1/33s are so close to 1/20s and 1/30s that they offer little practical benefit. Adding arbitrary values would make controls slower and more complex for very little real-world gain.

For shutter speed, modern cameras may internally time exposure very precisely anyway, especially in auto modes, but there’s usually no compelling reason to expose every possible value in the user interface.

For aperture, there can also be physical and communication limits between camera and lens, though historically some lenses did allow more continuous adjustment. Standardized marked f-stops became popular because they simplify exposure calculation and operation.

So the limitation is mostly about keeping camera operation fast, consistent, and useful rather than because 1/19s or similar settings are impossible.