Because to save costs, from the cost point of view, the more the same materials, the more expensive the cost, not only the product cost, including transportation cost and maintenance cost (cleaning time, etc. In fact, the tables in many restaurants are actually as high as at home, especially in western restaurants, but the tables in fast food or low-end restaurants are short, which reduces the comfort. I hope people who sit down will leave quickly (so are the tables and chairs in Starbucks).