Even with metal laptop cases screws usually go into metal inserts in plastic glued to the metal. If the case were machined with the metal of the case threaded, the case would be the most expensive part of the laptop and make the notebook cost many times more. One solution would be to use sheet metal the same material as the plastic inserts, thread it and glue it to the case. From what I’ve seen in notebook repair, even the most expensive notebooks still have plastic glued to the metal cases.
By the way, stressing the case can pull the metal inserts out of the plastic. Usually, the plastic is the first thing to break with stressing a metal case.