The real problem, aside from fixed aperture and relative lack of portability of scopes, is that scopes have a different optical design. While a good camera lens is also worried about the optical performance at the edge of the glass, scopes not so much. Emphasis is on performance at the center where a scope's subject is usually placed. 

For camera optics, a fixed telephoto always outperforms a zoom but the realities of travel photography has to include bulk (how many different lenses can you carry) and weight (both from what is allowed and hand-held vs. tripod). This is where a single zoom can be the best compromise. With digital cameras, every time you change a lens, especially in the field, you introduce a change of getting dust onto the sensor's glass. Other considerations include the maximum aperture of the lens (best performance is usually a couple of f-stops smaller than wide open), bokeh (the quality and shape of the blurred, out of focus parts of the image), image stabilization, autofocus speed and cost.

Another consideration is that while the desire is always for stronger magnification, especially with wildlife photography, lens performance still trumps. A cropped image from a good performing 200mm lens shot in RAW format at the highest resolution the camera is capable of still trumps a larger but softer image from a lesser performing 400mm lens.