Because Go doesn't do very much optimization. They've been fighting back performance regressions every time they try to improve the performance of the resulting binaries.
Neither does clang, gcc or rustc unless you explicitly tell it to. I was talking about a normal development session. Why would I make compilation time worse by making optimized builds?
https://dave.cheney.net/2016/11/19/go-1-8-toolchain-improvem...