Before industrialization, people used a millstone to press olives to make olive oil. Making olive oil at home today basically involves the same process. But not everyone has a millstone to their use. A mortar and pestle isn't really useful unless it's huge. The small ones for kitchen use won't get one far in terms of crushing olives. It will take too much effort and only so many olives can be crushed at once.
For those living in areas where olive production occurs, some farms may have the facilities for olive pressing and may be willing to press olives for a small fee. My grandfather used to send the olives from our olive trees to a nearby farm. He paid for the processing and in return we got excellent olive oil at very little cost to us.