Yes but where does it say the warlock gets the benefit of a special ability (eg magic resistance)?
It doesn't. The Monster Manual entry on the imp and pseudodragon talk about abilities they grant as familiars.
Technically, the warlock's familiar isn't an imp, pseudodragon, or any other statted creature. It is a spirit that can assume the form of certain animals, or that of an imp, pseudodragon, or sprite.
From that perspective it makes sense that it wouldn't grant you the abilities that the real creatures do...but that's kind of a nerf for the warlock's class feature (unless your DM rules that the real creatures don't become actual familiars, and therefore just die to the first area effect spell you come across, despite that fact that it is strongly implied that the
find familiar spell is necessary to form the bond with them, which implies that they should become actual familiars*).
So I'd say that you gain the abilities of the real creatures, including the special features they grant you as your familiar.
*In the warlock's case, there is even conceptual precedent with the Blade Pact. You can conjure a weapon of mystic energy,
or you can bond to an actual magic weapon. If you bond to a magic weapon, you can then send it off to never never land and summon it back at will. That's how I would interpret the improved familiars in the MM, whether acquired via the Pact of the Chain, or by non-warlocks who cast
find familiar.