Because First Class includes substantial catering.
I don't understand why this is relevant?
The railway doesn't have a way to differentiate between "substantial catering" or otherwise on a first class ticket. If I buy a first class LNER ticket then then the standard of catering is dictated by the train I'm on, as published on their website. I can't pay more money for a "Dine First Class" as opposed to a "Deli First Class".
The problem seems to be that Avanti have introduced an additional class of travel (which isn't properly supported in the fares data, but that is another matter) which sits between standard and first class. Since Avanti's additional class of travel is neither standard or first, it cannot by definition be comparable to either.
The other way of looking at it is if Avanti hypothetically introduced a class of travel above first class with, I don't know private compartments or something, and called it "super class premium". Avanti obviously wouldn't accept normal first class tickets in it.
So why should it work the other way around? First class means first class. If Avanti don't want to accept the industry-wide first class tickets (including in disruption) they should not offer the first class product.