Even if X1 is an online console only, I don't think any devs is stupid enough to make a single player experience limited by online connection quality, especially not if the dev expect the game to sell a lot. Being an online console only is one thing, being online console with a strict minimum speed and latency is another thing.
Of course there is the financial issue like Scott have said. In multi, the same calculation from the server is shared with the clients. In single player, each player basically reserving CPU time on the server, so it wouldn't be cheap to maintain, and I'm not sure you want to pay for the server time each time you want to play the game. The argument here probably since you already a gold member, thus MS should already reserve CPU time for you.
I don't know how much usually a server time 20x the X1 CPU cost, but probably not enough to be covered with $50/year subscription.