It takes a lot of manual work to produce a high-quality tea, except in Japan, where they have designed incredibly sophisticated machines.
Tea leaves are sorted one by one, like here, in China. This is done for any tea worthy of the name; in other words, whole-leaf, good quality tea. This leaf-by-leaf sorting eliminates tiny pieces of stem, as well as any coarser leaves. It is also an opportunity to remove the occasional insect: tea plantations are living environments, and the presence of weeds and insects can be a sign of good farming practice.