Why Golang projects seldom use mocking library in testing?

New to golang and is now researching on how to do test in golang. I see that there are popular mocking library like gomock. However, at the same time, I see that large golang open source projects don’t use any mocking library at all. For example, geth (ETH miner), wire (dependency injection library) and sendgrid go client (for sending email) all choose not to use any mocking library.

Considering the fact that there is no class in golang, is it a practice that not to mock interface (which most mocking library did)? Should I just make the function to be tested as package variable and overwrite in test instead?