Interfaces and helper types and abstract interfaces and that sort of thing don't actually run, though... it'd be a neat trick to get anything resembling the bug rate of code-proper out of assistive structures like that. Besides, not all languages feature that so heavily in idiomatic code—not everything's Java.

Anyway, that "bloat" is documentation your computer can understand so it can tell you when it's wrong. The alternative is documentation your computer can't check. Or just skipping that and letting the next person eat the higher onboarding cost and defect rate, which I suspect is more often the case.

[EDIT] And it's not unit testing instead. It's in addition to. One's not a replacement for the other.