It's simple mechanics.
Modern bolt-rifle design is much more interested in accuracy then speed of operation in a foxhole.
With that said, the old COC military rifles used a very heavy, very long, very slow striker propelled by a relatively weak spring.
Most every modern hunting rifle uses a very short, fast firing pin fall to reduce the lock-time, which increases the accuracy.
Trying to cock on closing when you hit a firing pin spring strong enough for a truck is hard to do.
For instance, the Lee-Enfield used a 13 pound striker spring, and it weighed 9 pounds.
The 93 Mauser used a 19 and weighed about the same.
Modern actions like the 700 Remington use a 24 pound spring.
Savage uses a 28.
SAKO uses a 30 pound spring!
Trying to cock on closing while compressing a very stiff spring, a very short distance, that is 4-5 times heavier then the rifle, is just not gonna work out for you so well!
Using a cocking cam with a high mechanical advantage built into the bolt body to do the heavy work is much easier & faster then trying to do it with the palm of your hand.
rc