Who maintains day.js? It‘s (i did not look, i guess) not the same people that develop the JavaScript standard library. At some point, day.js is abandoned or breaks something and nobody is going to fix it because it is abandoned, or even worse, soneone creates a fork and fixes it there and now we have 2 day.js.
If it is in the standard library, you have a guarantee that it works today, works tomorrow and with a quite high certainty still works the day after. There are not that many languages that successfully break backwards compatibility often.
And don‘t get me started with the legal aspect. There are many projects where one cannot simply pull another dependency just to calculate a date diff.
Regarding Moment, on their site and they plaster exactly why you should not use moment and explain exactly why other libs (Day.js included) are better. Node is designed to be modular, Java isn't.
A language should provide a set of functionality that makes the developers life easier and consists of a set that he can achieve basic (daily) things without the need of 3rd-party dependencies. And if that usually starts with „npm -i“ I‘m going to question that...