They are not monotonically harder, and they are not all the same exact skill being tested. So someone who has completed Project Euler problems 1-30 has done more to demonstrate her ability than someone who has only done Project Euler problem 30.

If your level is such that 30 represents your current challenge, then doing 1-29 won't take toooooo much time anyway. And you can still have fun trying to improve on the best solution offered so far for the problem -- there are multiple ways to solve a given Project Euler problem.