Why do ImmutableList.of() and friends prohibit null elements?

I explained this at the 25-minute point of this video: https://youtu.be/ZeO_J2OcHYM?t=1495

Sorry for the lazy answer, but this is after all only a "why" question (arguably not appropriate to StackOverflow?).

EDIT: Here's another point I'm not sure I made clear in the video: the total (across all of the world's Java code), amount of extra code that has to be written for those null-friendly cases to use the old standbys Collections.unmodifiableList(Arrays.asList(...)) etc. is overwhelmed by the total (across all of the world's Java code) amount of extra checkArgument(!foos.contains(null)) calls everyone would need to add if our collections didn't take care of that for you. Most, by FAR, usages of a collection do not expect any nulls to be present, and really should fail fast if any are.

In general in Google Collections the developers are of the group that does not believe that nulls should be an expected general purpose parameter.

From Guava's Github Page

Careless use of null can cause a staggering variety of bugs. Studying the Google code base, we found that something like 95% of collections weren't supposed to have any null values in them, and having those fail fast rather than silently accept null would have been helpful to developers.

The Guava position is largely, that there are other ways to avoid nulls in collections. For example, fetching a batch of items with a specific key. E.g.

// If a widget for the given id does not exist, return `null` in the list
private List<Widget> getWidgets(List<String> widgetIds);

// Could be restructured to use a Map type like this and avoids nulls completely.

// If a widget for the given id does not exist, no entry in list
private Map<String, Widget> getWidgets(List<String> widgetIds);