What does Henry Ford's quote Thinking is the hardest work there is which is the probable reason why so few engage in it' mean?

It means that people DON'T THINK. Some people are lazy and don't want to do it while others won't take the time to do it. Most people are taught as kids to memorize the correct answers, not to think. Another way to look at it - maybe the way Henry Ford did: It also points out to the fact that the people who do the thinking, typically the owners of large businesses, delegate most of the tasks that can be done by someone else, to devote the time to the most important of them all, thinking. This thinking involves how to take the next big step, rather than getting stuck up in the daily chores of running the business.