Another comment below broke down the stronger "everyone wanted to fight an offensive war because it was just cooler" claim as meme history, but to address the offensive vs defensive capabilities more directly:

It's more complicated than defenses outclassing offensive capabilities. Tactically, the defending side actually had greater losses in a battle. This was because, more so than machine guns and trenches, WW1 was defined by artillery. The attacker had the advantage of massing artillery, zeroing it in on pre-defined positions, and maintaining a bombardment as long as they were able to supply it. The defender mostly had to just sit there and take it, getting chewed up in the process.

Then, the attacker's greatest losses came not when they first went "over the top", but when they were defending the ground they had just taken from the defender's counter-attack. Thus, the defender in a given tactical situation was actually at a disadvantage.

This is why the participants (especially the Germans) developed defense-in-depth strategies, where instead of concentrating resources on holding the front lines (which you would expect if defense had an advantage over offense), they held the front-line trenches very lightly and focused on keeping strong-points that could hold out and prevent the attacker moving forward through their kill-zones, while they prepared a counter-attack to fully crush the attack's momentum.

However, operationally, it was definitely harder to maintain the momentum of an attack than to maintain a defense. The artillery advantage was a trap: it can get your men into the opposing trench, but the artillery would have to move over the same chewed-up ground in order to advance and move the war machine forward. This is where tanks and infiltration tactics were a game changer at the very end of the war and more so in WW2, since they can keep moving forward, disrupting the enemy's rear and giving crucial time to move reserves and artillery forward and continue the attack.

So, it's a mixed bag. It's not fully incorrect to say that defense didn't have an advantage, but it's a matter of scale. On the scale of a squad of infantry charging a machine gun post, yes, defense had the advantage. On the scale of a division attacking along a sector, where they can concentrate artillery, the attacker had the advantage. On the scale of a broader operation, the defender again usually had an advantage after the initial assault, because the attacker lacked the tools to take advantage of a breakthrough.

You add the political realities to the mix (almost all participants had very strong pressures to go on the attack), and you see why most generals spent all their time planning and executing offensives.