Coffee blending has declined and risen in popularity over the years, cheaper coffee blends can be a sign of mixing leftover or old coffee, however with premium coffee, it’s usually a very considered decision.
Large coffee retailers have a customer base who have come to expect exactly the same taste all year around, it's much easier to achieve such consistency with blended beans.
Blending beans also allows all the best qualities of single origin beans to come together to create a balanced taste.