Because obviously the game needed three-armed giants with poisonous saliva and penis-like toes (have fun unseeing that one) wearing one of the most ridiculous 'garments' ever. Current LA is +5.
They have fourteen RHD. Those are of the Aberration type; meaning you're stuck with fourteen horrible levels. What do you get in return? Mostly ability score improvements, huge size, and a bite attack. Oh, and you have three arms.
I find myself wondering what to use the extra arm for, though. Triple-wielding is bad for the same reasons dual-wielding is and using a shield without having to hold it in your hand is ridiculously easy. Even Fuse Arms won't work: it only affects creatures with an even number of limbs.
In the end, you're sacrificing fourteen levels of your build to gain a boring pile of strength, size, and weirdly placed limbs that will only barely reach +16 BAB. I'm having trouble seeing how that is worth it, and I'm definitely not going to add a LA to all that.
Final verdict: -0 LA.