Because people insist on playing him as a carry and he's kind of bad at it. He works best in an "anchor" role for your team - a hero you just throw into a teamfight and let them just sit there and line up the enemy team while your real damage sources knock them down.
If you go Soul Ring into Pipe of Insight (you could also get manaboots instead of Soul Ring, or even both if you really wanted to) with max Dragon's Blood by lvl 10, you have a hero with roughly 50% damage reduction to both physical and magical damage and something close to passive 20 HP/sec regen, without factoring in any other items he might have (Armlet, treads, whatever.)
DK can very quickly reach the point of being almost impossible to kill unless you can have your entire team sit on him for a good 10+ seconds, which isn't really possible in a hectic teamfight.
He doesn't really need damage. Abyssal Blade is good because it's good on everyone, Buriza is still the best source of damage in a single slot other than Rapier, blinker basically means your stun has 1200-1600 range. His role isn't really damage. You can build him as a damage hero, just like you can build Crystal Maiden as a damage hero, but it's not really what he excels at.
What he excels at is being incredibly difficult to kill, and that's what you should focus on. Soul Ring eliminates early mana problems, get a Pipe of Insight (enemy has nasty magic damage) or Crimson Guard (enemy doesn't have nasty magic damage) for your team, then get shit like Heaven's Halberd, Shiva's Guard, maybe even a sheeper. You could also get some damage like Abyssal or Buriza if that's what your team needs you to start providing.
And stop wasting gold on Lothar's. It's shit. It's shit on him, it's shit on basically everyone else, too. Buy a blinker. It's better for landing Dragon Tail without ulti, anyway.