That builds or doesn't an entire file. Assert works as a statement. There is not an equivalent in Go to conditionally removing just a statement in a function based on a compile time option.
That defeats the point of asserts. Now you have two copies to keep in sync with each other, whereas asserts are inline with the rest of your code and you have one file that can be built with or without them. They could use a separate tool to produce the assert free version, but that adds tooling beyond what Go provides. Nearly every mainstream language allows you to do this without any extra steps, except Go.