Why aren't nodes indexed when they are saved?
I believe it is in the interest of scalability. Imagine you had a large, popular site with user-generated content. Imagine you had users saving a node (say, posted a question or answer) every 20 seconds. Imagine a node being indexed as well as submitted every 20 seconds. The site would become unusable very quickly. So it's much better to allow the indexing to happen independently of node submission.