Let's not forget about how the UNSC decided that it was too practical to have warthogs be indestructible, so they decided to make them susceptible to damage. Originally, they had it so that if the Hog took too much damage, it would explode, killing everybody inside. Then they made a smart decision to make them indestructible, along with their tanks and other said vehicles. But, when they soon realized that these vehicles were too strong, so they reverted back to their original designs with explodable metals.