With Go 1 there was of course go get. With this it became a no-brainer to install code from arbitrary Github repositories. At least the social barrier is really high to properly put in a package at npm or PyPi. But they kind of democratized that.

Also when Go was quite new, many projects had no dependencies or at best a handful. I think that was a learning from other languages with large package repositories, namely Rubygems. People would create gems put them on Github and Rubygems with useful functionalities but abandon them after a few months. So many larger code bases were sometimes rendered unusable because the package became incompatible with new language releases or did even get removed from Rubygems.

Nowadays, since Go has risen so much in popularity and adopted habits from other Programming ecosystems, there are now Go modules which are on par or even better than solutions from other languages.

> why doesn't it have a proper package manager?

You should have a look at the release notes of the last three Go releases.

I'm not going to do that because I don't think Go is a good language and I have no interest in its ecosystem. I'm just here to give people a hard time.