The metal frame must be the key. I have an Osprey backpack without it, and the mesh alone does almost nothing to help with sweat, even though it was sold to me as being better. A friend in high school found an old Vietnam-era? US army rucksack with a metal frame that was less comfortable in the short term but not bad in the long, sweaty term.
[1] https://www.osprey.com/us/en/product/tropos-TROPOSF19.html