If a dretch lands a bite at the same instant that the unarmoured human starts unloading his rifle into it, the human will dish out 25 damage in 450ms, so the dretch will die 50ms before it gets a second bite in. So, if the human didn't start firing untill the drech was in melee range, and if the human misses some bullets at that range, only then will the dretch have a chance for the kill. That seems like the perfect balance to me - with 25 hitpoints and 64 range, suprise is still everything to a dretch.
If you think the dretch head shot from ground is overpowered, maybe you should test with a friend or with 2 computers to see just how far away you can actually hit with a non-headshot, or how far away you can hit the head while jumping. There is a huge downside to headshots from the ground, since you need to get much closer to the target to hit them.
That 2 frame gif in the other thread shows the max range of both attacks. Depending on how well your opponent is dodging, the weapon they're using and if they have light armour, it's often easier and safer to land two body shots for the same amount of damage.