The simple answer is "because they can." There really is no excuse for a multiplayer focused console game not to have local splitscreen, with the possible exception of console based MMOs -- although even Monster Hunter Tri has some sort of local splitscreen mode. Unfortunately, with more and more consoles being hooked up to the internet these days, the devs and publishers are able to get away with making people play online, since that's the mode they want anyway.
If I had to guess, I'd say the percentage of consoles connected to high speed internet has to be close to or better than the number of PCs connected to 56K or better internet around the beginning of last decade, and the only time people complained about online only multiplayer then was when the game was essentially multiplayer only, as was the case with Quake III and X-Wing Vs. Tie Fighter. Funny thing about those games: the single player was literally nothing but the multiplayer modes with bots instead of human players. Considering that those were some of the best games of all time, I really have to laugh at the people complaining about the 6 hour campaign in MW2. At least it has one.
Back on topic, the publishers love it, because that means everyone playing multiplayer has to have their own copy of the game, while most consumers don't mind, because it's rare that local multiplayer is an issue these days. Before I started college, I might have played splitscreen a couple of times a month. Now that I'm living on campus I play daily, assuming I have the time, but that's a result of having roommates who also game, not a result of a shift in my personal multiplayer habits.