They think it's sometimes easy to make money. The truth is that it is always hard to make money, and always will be. To see why, we can reverse it in our heads: do you think carefully about how you spend your money? Me too; and so does everyone else. That's the hump a product or service needs to get over before it can make much money. And it's true 100% of the time.
When a product or service looks like an easy way to make money, what's really happening is that it's a great product or service, so it gets over the hump more easily.
Another big misconception is that they think that ideas make money. They say, "there's already a great time-tracking app in the App Store," and think, "so it's not an idea worth pursuing, because it already succeeded."
The reality is that execution and operations are what make money--which means that many businesses executing on the same idea can all make money. How many dry cleaners are there in the U.S.? How many pen manufacturers? How many car companies? How many Mexican restaurants? Or car washes?
It's extremely rare that a concept is totally dominated by one large company. But because it is rare, those stories are endlessly told and retold within the culture. So if people are not thinking carefully, they can come to (paradoxically) perceive those rare types of companies to be the norm.
Here's a fun exercise: pull up the list of the Fortune 500 companies and see how many names you recognize. These are all huge businesses, but most people will not even recognize half of them. It gives you a sense that there is a big difference between being a famous business, and being a successful business.