We need the Web to be open, easy, flexible. We must follow this model because no one owns the web. In grocery stores we buy items in bulk. We do not buy them one at a time. Servers are just the same.
Good performance requires course-grained communication.
Servers have only a limited ability to multitask. A server cannot run more and more tasks. This is like having only so many cash registers in a grocery store. Even queues run smoothly until some problem task surfaces. For example, checkout clerks get interrupted to do a price check and the entire system experiences a slow down.