Well with Minecraft surpassing 150 million, Fortnite making billions, GTAV surpassing 100 million, Activision still making a ton of COD, EA even after the bullshit with SWBII having a great year, Pokemon GO being the most profitable Pokemon game, Valve making still a ton with CS/TFII why would you focus on single player games?
Because they're the best?
9 out of top 10 games on twitch are multiplayer titles.
Singleplayer games will be more and more niche as time goes by.
Even mobile games have a strong focus on multiplayer these days.