This is an excellent and frequently asked question!
With fast fashion taking over closets one season at a time, it has been very easy to take for granted the labour required to sew together one garment in its entirety. Out of sight, out of mind has been the motto since all production has been shipped off-shore. Mass distributed garments are made in places like China and India because labour costs are much lower and large factories are equipped to facilitate the mass amount of sewing required. Sewing 1,000 - 10,000 of one garment allows the price per piece to reduce and often the talented sewers are paid a fraction of what they should actually be earning. Sewing is a technical skill and requires training and extreme hand eye coordination to not only sew quickly, but also accurately.
These labour costs, high level of skill, and attention to detail are what you pay for when you stop in at your local tailor or alterations shop. It takes years of practice, schooling or apprenticing to achieve the illustrious status of tailor. Each tailor is equipped with an arsenal of tricks picked up over years of working.