It's probably not worth for them to so this. Why should they? They see the difference and the cockpits are probably different: wheel, pedals seat position etc. Not that easy to just jump into another car in these days.
And it would be a time waste for the team. They need to get that piece of crap up to speed rather than making kubica feels good or proving a theory to the public.
What I don't understand though is why kubica is throwing the car under the bus... Good pr for him but bad for the team!
Then why are the test and reserve drivers able to jump in and do it?