It does matter because single player is expensive to make. If you can skip it, you can cut costs significantly and still make a lot of money.
Yes, it is expensive to make. But so what?
If it is drawing in millions of extra gamers, then those costs are covered, making the issue entirely inconsequential.
But by removing it... Those that buy the games for the single player component may not buy the game, will not try the multiplayer, will not engage with the Microtransactions and other aspects.
It's like my local service station... My Vehicle is dual fuel, I primarily run LPG with Petrol as the secondary fuel.
They stopped selling LPG, citing lack of sales as they were only breaking even on it... Unfortunately that means I had to go elsewhere, which also meant that they missed out on Petrol, Food, Drink sales as well.
The result is... They have started to struggle even more, reducing hours, wages (I know everyone that works there) and so on. Why? Because they drove customers that were attracted for one product away that might have purchased other products.
Call of Duty is stupidly profitable anyway, Costs shouldn't even be an excuse leveraged.