For early stage startups, the board is usually only the founder(s) and investors. They all have received a lot of stock in the company anyway, so no additional compensation is necessary. Sometimes an outside board member is added, in which case they'll typically receive some stock options (similar to advisor shares).
Investors do not get money until the company is acquired or goes public and the shares the investors own can be sold for a profit.
Board members at large public companies do get compensated for their time.