Multiplayer focus is a cheap cop-out in many cases. Get some guns off an asset store or from previous projects, slap together some maps, instant game. I'll go out on a limb and say 90% of the big-ticket multiplayer titles operate on that principle. You don't need writers (yes, Bioware, you do need those), are lighter on graphics assets, voice acting can be restricted to a bunch of grunts and swears.
Producing a good singleplayer experience, especially a story- or exploration-driven one, is hard and resource-heavy, or at least you have to invest a lot more effort into rules and interactions. Of course some studios are working around that problem "elegantly" by shipping threadbare products in the hope that hordes of modders will fill them with life or in some cases even fix egregious bugs (I don't want to say Bethesda but oh yes). Adding working cooperative multiplayer to one of those makes things harder still, and antagonistic interactions all the way through would be on an entirely different level which is why it's often a afterthought, segregated into a separate game produced by the above principle. Something like a Wing Commander, Freespace, Heroes RPG, Psychonauts, Witcher, Blackwell, Monkey Island, Firewatch, Soma, Ultima, or Bayonetta (to name only a few) would be worse off for being a multiplayer title; Dark Souls with its institutionalised griefing definitely is.
Producing a good singleplayer experience, especially a story- or exploration-driven one, is hard and resource-heavy, or at least you have to invest a lot more effort into rules and interactions. Of course some studios are working around that problem "elegantly" by shipping threadbare products in the hope that hordes of modders will fill them with life or in some cases even fix egregious bugs (I don't want to say Bethesda but oh yes). Adding working cooperative multiplayer to one of those makes things harder still, and antagonistic interactions all the way through would be on an entirely different level which is why it's often a afterthought, segregated into a separate game produced by the above principle. Something like a Wing Commander, Freespace, Heroes RPG, Psychonauts, Witcher, Blackwell, Monkey Island, Firewatch, Soma, Ultima, or Bayonetta (to name only a few) would be worse off for being a multiplayer title; Dark Souls with its institutionalised griefing definitely is.