Shooting landscapes, you often have little say in what angle of view you need to use. You can't fly 20m into the air to suit the lens, nor can you walk off a cliff, excavate a hill or knock over a building to put your camera where your lens needs to be. Often, the spot you have is the best one you're going to get (because you scouted out all the vantage points on Google Earth before you got there). You need to make your lens fit the angle of view, not the other way round.
There are three ways to achieve this - crop, stitch or zoom. Cropping is out for anything less than an 80- or 100-megapixel MFDB. That leaves stitching and zooming. Stitching with primes works well for wider angles (where the ability to use the sharp, central part of the frame helps make up for weaker corners in almost all UWAs) but not so well at longer focal lengths, due to the number of large, heavy lenses you'd have to carry to cover all bases. This is where a zoom really comes into its own.
Landscape photography is uniquely suited to zooms - you need the flexibility of a zoom, but also don't often need the wide aperture of a prime, and most decent lenses, whether prime or zoom, will have similar performance at your typical apertures.