Don't shoot the messenger, but as I see it there are two reasons why people take the urine with carry-ons.
1. Many airlines discourage hold baggage by the fees they charge. Passengers are motivated by the airline's policies to check nothing in. Some are entirely free up to two bags (eg Southwest), but many charge for hold bags.
2. In the US, airlines are hub/spoke operations, so unless you live close to a hub, you will be connecting. People want to take their bags with them so they make the tight connections. A small delay on the first flight would very possibly mean that checked bags would not make the second flight. As an illustration of this, the crew's bags are always in the cabin, never checked. If the crew's bags are important, so are the passengers'.