Gaining Google's trust doesn't happen overnight. It takes time. Think about building up your relationship with anyone. The longer you know that person, the more likely that trust will solidify. So, the reasoning is, that if Google just met you, it's going to have a hard time trusting you. If you want Google to trust you, you have to get other people that Google already trusts, to vouch for you. This is also known as link-building.
Just think about any relationship for a moment. How long you've known a person is incredibly important. It's not the be-all-end-all, but it is fundamental to trust. If you've known someone for years and years and other people that you know who you already trust can vouch for that person, then you're far more likely to trust them, right? But if you've just met someone, and haven't really vetted them so to speak, how can you possibly trust them?