When you are writing code, you generally know what it is you are trying to achieve. When debugging code, you're frequently trying to find out why a problem is happening in the first place; often in code that someone has written or that you wrote months or even years ago.
I'm not saying that debugging isn't a skill you can learn, but it's a superset of writing code, so it's by definition harder.
But being harder is not even about the skills themselves, it's about mental effort it takes to do something. And designing and implementing things is certainly much much harder, than digging into something already designed and implemented.