I think it’s because CS chose to continue with a ‘restaurant car’ style operation which was based around an a la carte menu on which all items were individually priced, and which (importantly) allowed standard class passengers to dine. In the 2000s GNER (as was) and Anglia on the London-Norwich route did not offer free meals in first class (only drinks and snacks) but provided restaurant cars which were available to both classes, although the meals were definitely not cheap. This is similar to GWR’s pre-pandemic operation with the Pullman. East Coast (as it became) and Virgin went down the route of free meals for first and just a buffet for standard.
The reasoning behind CS’ decision was perhaps was that first class passengers would be happy to pay for a higher quality offering with more choice. In the past they did experiment with meal deals and even £5 refreshment vouchers for first class, but these did not last. Perhaps CS could say that first class passengers get a main course, dessert and drink included in the ticket price, but then anything else on top of that would need to be paid for. Otherwise CS will end up giving away a lot of quite expensive food, which would make the lounge car uneconomical to run.
Another point is that not every first class passenger on the sleeper requires food- many will already have had their evening meal (especially on the Lowlander) and will require only minimal breakfast; the En-suite room will be a major attraction for many.
My two wishes are that CS dropped the silly marketing speak and called a spade a spade (to me it will always be first class berths and the lounge car, never club rooms and the club car, probably because I was so used to the old stock), and that they had perhaps lowered the fares slightly during the pandemic period when many of the facilities are unavailable. That said, the government restrictions are not CS’ fault, and the operation is already massively subsidised. At the end of the day, you are paying for a near 600-mile rail journey to Inverness or Fort William (for which I would doubt you could get much change out of £100 even in standard class on day trains, even using advances) and for the convenience of being able to travel overnight rather than wasting a day of your holiday or your working week travelling. For many people that’s the main selling point of the service, and that can still be fulfilled even in these pandemic times. If CS are now able to fill the Inverness and Fort William trains on Friday nights, even with the ongoing restrictions, then they’re obviously doing something right. As has been said above, there is no need to introduce perks, gimmicks or discounts if they can fill the train.