Assuming you have a flexible ticket, which the overwhelming majority of passengers don't.
Tried asking BA/AA to move to an earlier flight on a non-flex ticket? "Computer says no, unless you pay $lots"
Sure, if you're flying on the "cheapest flight on skyscanner" then you don't get the flexibility, but that's why they don't fly 6 A380s rather than 12 smaller planes, they want the £6k tickets to choose BA/AA rather than VS/DL. The £600 return tickets at the back will fly whoever's cheapest.
That's a sweeping claim.
Pick a highly-discounted business class fare LON-NYC-LON for dates well in advance which books into I fare bucket.
I close the ILX1C1S6 fare, out 25 Dec 2019, back 1 Jan 2020. (The base fare is GBP1271 r/t...)
Either do a dummy booking at BA.com, or go to ExpertFlyer[0] and the fare rules. Find the 'PENALTIES' section:
CANCELLATIONS
ANY TIME
TICKET IS NON-REFUNDABLE.
CHANGES
ANY TIME
CHANGES NOT PERMITTED.
"Almost all" then, especially when you factor in corporate deals
At BA.com I've just looked up LON-NYC-LON out 6 Mar back 13 Mar (so just over two weeks from now), and I'm seeing the same complete lack of flex:
Your ticket conditions
Changes to your ticket No changes permitted. No upgrades permitted
Refunds There are no refunds except for any government & airport taxes
No A380s on this route. There are 32 flights each way, 28 from Heathrow.
BA/AA operate 12 daily flights LHR-JFK, pretty much one ever 90 minutes during the day, it's pretty much a turn up and go service. Virgin/Delta do another 8 flights, again turn-up and go.
That frequency attracts the people on flexible tickets. Those on the cheapest flights they find on skyscanner will probably take the norweigan 787s from Gatwick.
That market for flexibility doesn't exist between say London and Singapore (I suspect most are transferring at least at one end), but there's plenty of passengers looking to travel, and so you get 4 of the 7 flights over 3 carriers are on A380s.