Div-3 problems are easier to solve, but that does not mean they are easier for problem setters.

At first, there is some work have to be done for every problem, even A+B. You'll have to write and test validator, checker, and the correct solution very carefully. Just writing them like solving a problem in contest won't work: for example it should be forbidden to use scanf in a validator or checker, as it invokes undefined behavior for bad inputs. And you'll have to find a group of people to test the problem. The construct of test input is tricky: randomly generated inputs aren't sufficient for many problems.

And it's actually more difficult to make a problem suitable for less experienced participants, as the problem has to be creative and interesting (i. e. it shouldn't be a duplicate of a well-known problem, or the contest will become "Google contest": test who can use Google better :).