because they already are. an industrial arm from ABB is frequently over $100k. add in the cost to fit it with specialty equipment like vacuum suction for handling boxes, made by a small to medium size business, they'd probably charge another $50k. and if it breaks you need specialty mechanics and parts.

in a world with 500 million humanoid robots, parts are plentiful, theyre easier to work on due to not weighing 5000 pounds, and like the other person said, economies of scale