Because engineers can't talk to customers. They need a project manager to receive the requirements sent over from the customers, and then bring the requirements to the engineers (or have someone bring them over).
Edit: Having been around for a while I’ve learned to appreciate PMs. You have to be very careful what you say in front of customers or focus groups. If you had two engineers and one customer chatting unsupervised it would turn into a brainstorming session, and after 45 minutes we’re building a flying police box that can travel through time.
Sometimes even asking the wrong question can cause an upset. Engineers are prone to thinking or even saying things like, "successfully delivering the project is more important than the ego of a middle ranking procurement manager in the client, right?"