Is it standard practice not to include an ‘offline’ mode for a web application?

There are many discussions around the topic of accessibility, but taking the work accessibility in its pure sense of having access to the system, I am curious as to whether designers simply make the assumption that network access is standard and therefore there is no need to create an offline mode.

The primary scenario where I have seen this being implemented is in games where you need to connect online to access community or multi-player features, so they differentiate by having a single player mode, but many still connect to the web to save data or access information.

Is this a technical consideration around being able to save and process data locally versus on the server side, or is it just a given in most use cases that network access or interruption doesn't happen? Is this actually a reasonable assumption to make or should we always try to provide an offline mode by default?