Easier adoption on platforms where GNU make is not installed by default. Examples: Windows, MacOS, ChromeOS, FreeBSD, OpenBSD, 9front.
Exercise Go philosophy (extreme simplicity).
Reduce code size & dependencies. Important for security. Since this project is used in network-facing applications (building websites) attack surface should be minimised. Yes, this software does not directly communicate with the network, but local exploits are also important.
Using make is for the sake of simplicity, I'm not going to remove it.
If you don't want to use it, then you can run the go generate, go build and go test commands, just as the makefile goals do.
Why: