Personally, by complexity I refer strictly to combat complexity. Asking why people like combat complexity is like asking why some people prefer C++ to Python, it just happens. I like combat complexity because it adds variety and I like variety.
Was not actually responding to your post, however...
when mob variety is reduced
It largely depends if the reduced mob variety has more individual variety. Just because you have multiple types of mobs does not mean more complex. Hell, I would say most of the time, it just means there are different types of mobs.
tactics get simplified
Simplified tactics does not mean less complex if there is a lot less time to react to your surroundings, in fact you could argue it actually increases the tactic choices, even if they are simplified.
Either way, it largely depends on preference and not everyones preferences are going to be met, the only thing you can do is judge them based on what they are and not what you want them to be.