Most businesses require startup fees as well as a cash flow to finance the products being sold. However, affiliate marketing can be done at a low cost, meaning you can get started quickly and without much hassle. There are no affiliate program fees to worry about and no need to create a product. Beginning this line of work is relatively straightforward.
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.
There are lots of Amazon affiliates in this space, mostly because it’s a strong hobby (i.e. passion) niche with tons of products. Which is good, since tons of these big stores are very general in nature, making it possible for big outdoors sites to possibly even pick one or two stores to recommend (this, in turn, might have other advantages, too, like negotiating better rates with those brands).