Because Horford is the classic low risk, low reward guy.
He's not going to shake things up, he's not going to cause locker room problems, he's not going introduce any major flaws or liabilities on either end of the court, he's not going to demand trades or say stupid things in the media.
But at the same time he's not going create offense, he's not going to anchor your defense, he's not going to control the boards with any kind of consistency, and he's not going to do a whole lot to move the needle - or to take you any closer to realizing the dream of becoming a contender.
You could say that if Durant is Batman, then Al Horford would make a very nice Robin. He'll give you all the support you need, but he lacks the talent and motivation to every risk trying to take your spotlight - he's happy enough living in the shadows.
It's not bad having a Robin on your team to handle some of the little tasks - it makes for a nice distraction for your enemies, which helps Batman focus on doing the heavy lifting with minimal interuptions.
My problem is when Gotham City decides to pay Robin a Batman-sized salary, leaving the city with no money left to actually sign a full time Batman. Then you get stuck depending on Robin to be your big hero, and many years of suffering inevitably follow.