An interface being equal to nil just because the value it contains is nil would be even more confusing. For example, imagine if you had two interface values, one holding a nil * T and one holding a nil * S. They would both be equal to nil, but not equal to each other, losing transitivity of equality.

Also, sometimes nil is a perfectly valid value of something and can implement interfaces. You can call methods on nil receivers because method calls are just curried function applications over the value. It would be odd if the interface compared equal to nil just because I happened to implement it with a value that's valid with nil.