Yes a single team could decide on a custom style, however when you need to cooperate with other teams, it takes a lot of wasted time in politics to agree upon a style.
It's also harder to read random code written by others, e.g. open source libraries. You can't just glimpse at it in the browser, you have to download the code and run it through your custom style applier.
THIS.
This is not stated enough in the value of gofmt - it's the standard formatting style for everything. I've worked in Java and C++ for a couple of decades, and while every team has had its own style guidelines, and occasionally tools to enforce them, even different teams in the same organization would have difficulty understanding each other's code.
The value of gofmt is that it's the only formatting standard, thus I never have to learn a new formatting style when reading the code for an open-source library, or coming in to a new job. It's exactly the same as what I've been reading for the last 3 1/2 years of working in Go.