Because creating a boss that's hard because numbers is easier than creating a boss that's hard because it actually challenges you, as a player, to use your skills creatively in order to beat it.
DS2 is pretty hit or miss in that regard.
For instance, the Mirror Knight fight is a beautiful and amazing affair, it challenges you in all the right ways, and the fact that, if you are online, one of the mirror shadows can be another player just ADDS to the experience. It's not really unfair (unless you get a hacker), and it integrates PvE and PvP in a very organic way. Very cool, awesome experience.
The Flexile Sentry fight, too, absolutely incredible. It's not gimmicky and doesn't has a lot of artificial difficulty, and there's a subtle time pressure on you to finish the fight relatively quickly, because the water level is rising throughout the fight. If you wait around for too long, soon you'll find yourself unable to roll because of the water's depth, and then unable to run, and the fight gets progressively harder, as the environment is detrimental to you, but not to the boss.
But, then again, there are other bosses that are just... Well, they're boring. Giant Lord, i don't know if it's because i killed him over 9000 times to farm souls, but this fight is seriously, seriously boring. And then there are the fights that are just unfair. Deck's stacked against you and there's little you can do but to push through, grit your teeth and stare at the "YOU DIED" screen over and over and over and over again until you finally get through and move along with the rest of the game.
I think the first Dark Souls, overall, had a better balance in terms of artificial difficulty, boring fights and truly uphill battles. The single worst offender in terms of artificial difficulty, in my opinion, was Capra Demon, because of the two dogs and small space in which you fight him. Stray Demon is also a bit too powerful for when you first get access to him, but as he's completely optional, and after you die to him the first time you can completely avoid him if you so choose, i don't feel it's too unfair, plus, you eventually need to fight Demon Firesage which is even tougher, so, by the time you're strong/good enough for that, you can go back and deal with it.