There may be other stated ancillary reasons: social gaming, cloud enabled, anti cheating, but the main advantage always online has is Digital Restriction Management. The big dev houses and publishers are all over this. Given the current state of gaming, there isn't anything we (the collective we) can do about it. The EA's of the world have all of us by the short hairs and they know it. There is no viable market reason they will change anytime in the future.
Reason? There are way too many gamers that will still buy the product. These gamers range from too young to care to gamers who buy the game while grumbling about always on restriction and gamers for everything inbetween. The only way to change this developing trend, is to hit them where it hurts. Don't buy their game at all. You could take the time to write them a dead letter about why you refuse to buy their product, but they will relegate those letters to the looney trash bin.
So enjoy the always on DRM. Maybe technology will catch up and nearly everybody will have gaming friendly Internet connections in the future. This is unlikely in the next couple decades. So there may be a chance where the new generation of gamers will discover why DRM is a bad thing for single player games. Thus adding to the percentage of those that refuse to buy the game. But even then, a new set of gamers that are completely unconcerned will come into the market. It's a losing battle.
I wish the prospects were better, but with the instant gratification gamers types are in general - the Publishers will keep a firm grip on the bit and we'll go where they tell us. And we'll like it dammit!