Yes, I thought when I was starting that 1u and 2u form factors were to save space. Maybe they are, but they also have the advantage of moving air front to back very effectively through and over the components. Though there still must be some need because I see even those boxes have optional manufacturer built memory shrouds to try to force airflow between the DIMMs.

I had some 120x38mm fans from another server box that I pulled out because they were too loud and I didn't need the static pressure they were giving. They went in here. That 13mm (and the extra 1k rpm) moves so much more air.