Even people who started out by grabbing everything they can get learn to be selective with their clients later. This is just enlightened self-interest -- small companies can only work with a limited number of clients every year, and you want those projects to do well and the customers to be happy with you. This is simply because this is a word-of-mouth, relationships based industry. Every project that does not do well is an opportunity lost in building a healthy pipeline down the line.
This dynamic however goes out of the window as the company becomes successful and becomes brand-driven - customers reach out because of the aura of the company and not necessarily because a friend of a friend talked about how you once helped save their company from a cliff.