The tasks you do to recover from the failure is often different from the happy path. For example, the happy path of dumping garbage is carrying a garbage bag to a collection bin. The non-happy path is that the bin is overflowing and you have to put the bag on the ground, or if the bag leaks and you need to move to a new bag, or if the bag breaks entirely and you have to pick up the trash again.

But yeah, I think a better way to put it is that sampling the happy path would indeed make the failure case easier, but sampling just happy paths is far from sufficient from completing even some of the simplest human tasks with failure.