You can, if you want, opt into warnings causing your build to fail. This is commonly done in larger projects. If your merge request builds with warnings, it does not get merged.
In other words, it's not a bad idea to want to flag unused variables and prevent them from ending up in source control. It's a bad idea for the compiler to also pretend it's a linter, and for this behaviour to be forced on, which ironically breaks the Unix philosophy principle of doing one thing and doing it well.
Mind you, this is an extremely minor pain point, but frankly this is like most Go design choices wherein the idea isn't bad, but there exists a much better way to solve the problem.