What is the complexity of calling a trivial hash function $n$ times?
Your question is ill-defined, and it's precisely that that I believe to be your misunderstanding.

There isn't an inherent cost in Landau notation, or any complexity analysis measure for that matter. You first need to define a model, by defining exactly what consists of an operation.

For example you could model only assignments, in which case the complexity is $n$. You could also model only hash function calls, in which case the complexity is also $n$. Or you could consider both to be one unit of time, in which case the complexity is $2n$.

Or perhaps your hash function is $100$ times slower than an assignment, which we assign cost $1$. Then your complexity would be $101n$.