All things being equal, a fixed power scope is better than a variable at the same price.

But all things aren't equal. Buyers want variable scopes, so that is where manufacturers invest in improvements - and cut profit margins to get volume sales. So the 3-9x40 at roughly the same price as a 4x40 may have design and quality advantages.

For example, I can think of one prestige manufacturer whose fixed powers have been basically unchanged for at least 30 years, whereas they bring out new variables every couple of years.

By contrast, among binocular enthusiasts (eg birders) the received wisdom is that variables are an inefficient gimmick. So the investment stays in fixed-power binos.