The Space Shuttle Main Engines are not used only once. Their design life is measured in seconds, and is around 50 missions worth. They are inspected between missions, and any worn parts are replaced.
They were originally specified to be usable for 100 missions each, but to increase the cargo capacity of the Shuttle, the maximum allowable thrust for the engines was increased to 109% of the original design, which shortened their life.
The Shuttle's basic design is part of why it costs so much money. A new external tank has to be built for each launch, and the solid rocket boosters, while they are reusable, must be towed back from the recovery area, disassembled, cleaned, inspected, reassembled, and reloaded.
A "single stage to orbit" vehicle would do away with all that, and would be far cheaper and simpler.