>Why would the compiler import (that is, parse and compile) a package that's unused in the code?
Parse - yes, compile - no. Compiler still needs to parse your code to determine if your import is used or not.
Go team decided that compilation should take as little time as possible, so it is easier to make sure that the compiler does not have to worry about unused imports at all.
Not to mention that this keeps your code clean.
Dozen separate tools can be used effectively in your workflow (say CI\CD pipeline). Or just executed on every save. (or both)