Because features in software don’t exist until someone adds them.
...because x doesn't have y yet....this stuff doesn't build itself, and it certainly doesn't get built overnight.
To suggest generics weren’t here sooner because no one wanted to make the pull request it is just dumb.
the go project is pretty upfront with how they go about deciding what will/wont get into the project, what process to follow, etc.
Posing it as "why doesn't go have generics" is bound to be reductionist, because it's too coarse of a question, and any real implementation winds up having a lot of nuance.
the question winds up just sounding entitled and petulant though, so if someone can't be bothered to ask a well informed question about why go doesn't have generics yet, the best answer really is "because it hasn't been added.", tautological as it may be.