Maybe because those game franchises are great enough without a unneeded multiplayer component.
I don't really get this attitude some multiplayer enthusiasts have towards single player only games,those who prefer to play alone aren't incapable of having friends,nor are they introverts.
I will say this for the first and only time:Not every single game needs multiplayer nor does every gamer feel like socializing.Let them have their games and enjoy your multiplayer,there's no need to force your preferences on those who don't want to or like playing online.
Bioware's developers actually like multiplayer, and have a long standing history of including it into their games. There was actually quite a backlash when they announced that MP had been cut from DA:O - a lot of their fans claimed that they were turning their backs on them. You might not think MP is worthwhile, but that doesn't mean that it should stop the Bioware developers from doing what they want. They didn't create MP in DAI because someone forced them to, they did it because they wanted to.
1. That still doesnt mean it doesnt affect the SP quality. Or some elements. As others have pointed out with the mapping of abilities or the amount that can be used, the UI, perhaps balancing.
2. Being that DAI is coop only, I would imagine that it would lose peoples interests since it lacks competitivness.
3. I know that wouldnt be able to answer that. I never said they need to be MMOG to be more fun, but the subject has been brought up before so it was just asked. Kinda like the testing of the sneaking around aspect MoTA DLC. It was fine and interesting because it was new. But lets be honest, the quality of it pales in comparisons to other games.
1. It may have had an effect, but it is much more likely that it would be an overall positive effect than negative. They can share technology and assets that would otherwise have to have been duplicated, and that means more content can be created more efficiently. In all honesty, the switch to Frostbite from Eclipse is the largest likely culprit for any UI/controls/etc. issues, much moreso than multiplayer development. I've actually worked with Frostbite before, it's a bit of a bear to get used to.
2. Divinity: Original Sin, Diablo, and ME3 are also primarily played as coop. They are widely regarded as excellent games and still being played today.