interface{} doesn't give you generics. The point of generics is to give you two distinct things:
1. You can write code that performs identical logic for a range of different data types, without having to know what they are.
2. That code can be checked for type safety.
interface{} gives you half of 1. You can write code that performs common logic and takes an interface{}, but then that code has to manually switch on type, or convert type before running.
Which gives you trouble with 2. Once you don't have a distinct type, you can't check for safety. All the compiler can do is check that an interface{} is passed in; which is a pretty weak guarantee. Meanwhile, in languages with generics, the you can write a generic method that uses the + operator and the compiler can check whether it works for numbers (OK), strings (OK), HTTP servers (uh oh, stop the bus!)...
1. You can write code that performs identical logic for a range of different data types, without having to know what they are.
2. That code can be checked for type safety.
interface{} gives you half of 1. You can write code that performs common logic and takes an interface{}, but then that code has to manually switch on type, or convert type before running.
Which gives you trouble with 2. Once you don't have a distinct type, you can't check for safety. All the compiler can do is check that an interface{} is passed in; which is a pretty weak guarantee. Meanwhile, in languages with generics, the you can write a generic method that uses the + operator and the compiler can check whether it works for numbers (OK), strings (OK), HTTP servers (uh oh, stop the bus!)...