For the same reason tractors and trucks are built for heavy loads and racing cars are built for speed. When a lot of DEAD weight is put on a wheel the vertical and side forces are significant but more importantly a replacement wheel isn't available at the end of the ride as would occur for someone riding a racing mtn. bike then driving home.
Think of it this way, when a rider leaves a bike on a downhill crash most of the load on the wheels disappears, on a loaded touring bike it's always there. Sure a 719 is a strong rim for the forces experienced while RIDING, but if you have to toss the bike on a truck and someone walks across your wheels you might want something beefier. Durability isn't just what happens while riding, it's everything.