Actually the battle intensity was bigger in WW2 than in WW1 but the "battles" of WW1 with their established time frames were just longer in general. Operation Zitadelle for instance had 220.000 casualties in mere 10 days while the Battle of Verdun, one of the most iconic battles of WW1, had about 800.000 casualties over a span of 300 days.

But the bloody casualties in the western theatre were considerable lower than in the east. The Normandy for example saw "only" 70.000 KIA over 80 days. Compare that with roughly 250.000 KIA within just 50 days around Kursk. The reasons for that are complex but big factors are different tactics from the Red army and Western Allies and the big willigness of German troops to surrender to Western Allies especially in low quality troops.

so your observation is only partially correct, the Western Front in WW2 had considerable less bloody casualties than the Eastern Front which might give the impression of less intensity but overall WW2 saw combat with higher intensity and often far higher casualty rates.

To build on what /u/ChristianMunich has said, the Battles of WWI especially on the Western Front, were of a much greater intensity and duration than their WWII counterparts. Only the Eastern Front in WWII comes close; also consider that the T3R (Tooth-to-Tail Ratio, Combat Personnel-to-rear area personnel) was much greater for WWI divisions than WWII divisions, and so there were simply more people on the battlefield to be killed/wounded. Moreover, Normandy wasn't exactly a picnic; according to Gordon Corrigan in Mud, Blood and Poppycock, the death/loss rate per division per day for the Allies on the Somme was 113, while in Normandy it was 99. I'd also hesitate to compare July 1st 1916 to June 6th 1944, considering July 1st saw c. 18 Allied divisions pitted against 7-10 German divisions, while June 6th saw much less forces involved, with the Allied forces possessing much greater firepower in terms of armoured support, air support, naval gunfire support, and artillery support. Heck, even at the Squad level 1944 infantry had more firepower than 1916 infantry, and more planning went into Operations Neptune/Overlord than into the Somme Offensive.

What about disease within the trenches? Didn't the Spanish flu outbreak and other diseases not directly related to war wounds prove incredibly devastating?