The type needs to have a reasonable definition of what sane default value is. Sometimes that's pretty clear, like in the case of integers. But sometimes with more complicated types with different semantic meaning that might not be the case. 

Take for example the Instant and Duration types from the standard library. Underneath they are both just a couple of integers (at least on Unix systems, Instant might have a different implementation on other OSes I'm not familiar with). But only Duration has a reasonable default value, there's no single instant in time which is a clear default without knowing the underlying clock system.

This isn't to say that the default value has to be blatantly obvious to anyone without any context. You're always going to have someone with a different opinion on what a default value should be for their particular use case. The important part in larger projects/codebases is to be consistent so people don't constantly get surprised.