You're probably aware that the high-end spotting scopes can be acquired with zoom lenses and that most offer single-power lenses.
In the past, it was widely accepted that zoom lenses didn't provide the resolution and clarity that the single-power lens did.
I remember 30 years ago purchasing a spotting scope with two single-power lenses for that very reason.
And it remains true that single-power lenses can be purchased for most spotting scopes that offer considerably more field of view than that of the zooms.
But zooms have gotten so good that image quality isn't the problem it used to be. In fact, modern zoom lenses are so good that it would be my first choice for most uses.