If your internet isn't very good now a days odds are your not buying games either, or paying for memberships. So there isn't a point to catering to them because they don't have as much spare money to spend on video games.

From what I've seen the quality of internet connections depends completely on where you live, not how much disposable income you have. Especially true in areas where there's only one ISP available (way more common than it should in the US from what I've heard), those tend to have it the worst since there isn't any competition so the ISP can get away with pretty much anything.

That money is usually better spent on multiplayer DRM, marketing, yet another content distribution network, or more preloaded DLC's.

ftfy

At first glance, you might think that games like COD would already have a decent AI implementation for their Single Player Campaign, at least that was my first thought.

But these implementations are probably less sophisticated than you think: the only thing they have to do is run to cover and shoot at you. And maybe a very trivial "spotted" method for stealth missions. Everything else, any tactics behind it, is probably done via level design, backspawns for flanking etc. So multiplayer AI takes up a lot more resources than you might think.

The last game I played that had multiplayer AI was Counter Strike, here you really don't have a Single Player Campagin. Multiplayer matches are sort of the only thing. On the other hand you have games like MOW/COD that have a full blown SP Campaign, the single players are likely already satisfied with this.