We already know that Oscars aren't the definitive measure of success. If they were, then Brokeback Mountain would've won in 2004. But regardless of the flaws and philosophies behind award shows, an Oscars win does mean something for a person's career. First, it means studio marketers can put "Oscar-winner" in front of their name on all future movie posters. It also means they've been formally recognized for their talent.
Given how prolific and talented they are, you'll be surprised to find that these actors haven't yet received Oscars. Leonardo DiCaprio sure is happy to be off this list.