> Which still doesn't answer the thread starter: why are samp, kbd and var 
> specifically still in this basic grammar, when they only refer to 
> specialised types of content in a particular field?

Their correct use isn't actually particularly narrow, almost any
user instruction manual for any piece of modern technology will use
all of these constructs, although samp and kbd may actually contain
graphics which, though standardised, may not have found their way into
Unicode yet.

Also web sites are creations of computing technology so the help
pages for web sites are particularly likely to require these constructs.