Because they don't believe they can, there are no limits with LOA. The only limits with it are the ones we put on themselves. The universe isn't there judging, it isn't biased, it doesn't tell you what you can or can't have, it brings you what you think and believe. People view manifesting a specific person as a hard task and can't align with it so they assume they can't. The universe doesn't deem desires or big or small, in theory, it takes the same amount of effort to manifest a specific person as it does a bottle of water, we just believe it's easier to manifest a free bottle of water and feel less resistant towards it. It's all about what you think and believe, if you're 100% aligned with any desire there's nothing stopping it manifesting it instantly. 

The belief that you can't manifest a specific person, or only if you are lucky comes from the idea that there is only one world. People have their old view of the world with solid trees and tables and people who act in uncontrollable ways and have free will. Then, they hear about LoA and try to include it in their old world and that just doesn't work. And then, all the questions that LoA newbies confuse come up. "Does that African child choose to starve?", "Who gets the job, when two people use LoA to get it?", "Is there free will?". But there isn't only one world. Creation is finished. All possibilities already exist. You don't really create anything, you align with the version of you world you vibrate at. So, just because you see the child starving, it doesn't mean that his consciousness experience the same reality. When two people apply LoA properly to get a specific job, they both get it in their reality. Yes, there is free will. You can always choose what you want and when your want something different than another person, you shift to a different reality in which the person wants the same thing.

You decide what is possible and what is impossible, if they think they can't atract someone specific then they can't, i f you think you can, then you can. 

Just ignore those coments and make little experiments to see how it works