It is a lot easier to control dimension than weight. People can put the small but heavy items in their pockets when their carry-ons are weighed.

While there are many people like to overpack their carry-ons, there are also a lot of people who don't. I checked my "carry-on" about 50% of the time. It was only 6.5kg (my allowance is 15kg) the last time I did that, and that was my only piece of luggage. I like to travel lightly, and do not want to struggle with the space in the overhead bin for my luggage. I also like walking inside the terminal, and prefer not to carry anything with me except my laptop bag.