[cinnamonheart]

It depends on the audience and goals for your language. If your language is intended to be a playground for new ideas, like a new type system or semantic model, you may only be concerned about getting that working and into other peoples' hands for experimentation, rather than usability.

[UglyToad]

I can certainly see why you'd not take the time to/need to do it if the purpose of the language is to explore concepts in language design.

But it seems to be missing from a lot of languages that launch with the intention of being used for large non-trivial multi-user codebases.

My question I guess was intended less as a criticism of language designers for not including these features and more to find out what I'm missing, that I'm not able to remember the entire (for example) Python standard library, or even the name of methods and classes I previously implemented.