Defense wasn't stronger than offense in a physical sense. It's just that the proper doctrine and training for dealing with the new technological reality was not developed until the Ludendorff offensive in the last year of the war.

This dynamic crops up pretty often in historic wars, in which it's less the technology that determines the result but how well the humans are trained. The Civil War in the US was also a slog because of the poor level of training among both enlisted and officers. There was a tendency among poorly trained or untrained volunteers to 'go to ground' to hide from gunfire. Neither infantry nor cavalry really learned how to charge properly because of deficiencies in training, which made the battles go on for a lot longer than they would have otherwise.

It was a similar situation with WWI. It is very, very complicated to coordinate artillery and infantry and to properly plan a successful penetration through fixed defenses. This came to be more or less a solved problem by Germany under Ludendorff's new doctrine.

A lot of the breezy summaries about WWI about it being static also only apply to the western front, because there was lots of successful maneuver warfare in the east. That's why Ludendorff got the job to try to end the war in the west after his success against Russia.

We see this every day with the technology industry: often we have a technology that works well years before we can get people to use it properly or to make it user-friendly enough for everyone to understand readily. Coordinating artillery with infantry movements required lots of work in math/statistics and drilling plus lots of data collection to make the plans and timetables realistic.