The Soul-Force Effect

An odd phenomenon which essentially boils down to "The more degrees of separation from force input by a soul, the less effective a weapon seems to be". In other words, crossbows are inherently just less effective than bows, even when theoretically firing the same projectile at the same velocity and rate of fire. Attempting to quantify this effect is challenging due to the inherently subjective nature of the green quanta that it stems from: it's a soul-based effect which only affects other souls. The fact that testing it inherently modifies its value makes it extremely difficult (if not impossible) to establish baseline values.
While not formally named and truly understood, most cultures subconsciously recognize this.
Note that this effect does not mean that a cannon projectile cannot kill a person. One way to look at it is that if the universe had aligned so that the person was positioned in such a way that they would instead only be partially crushed, that instead will happen unusually often.

Theory

Souls inherently resist what they perceive as damage. This effect is more prevalent the less soul energy that the other acting force has. The exact degree to which damage is resisted is very hard to determine or quantify. It appears to be that Celestially uncharged actions affect a baseline human around half as much as an act directly from another baseline human will. Measuring the effect for higher entities is even harder (if one had the opportunity to in the first place), as separating the soul-force effect from their automatic defenses can be a challenge in itself.
The prevalent theory is that all actions performed by souls (i.e. sentient beings) are inherently charged with green quanta linked to the effort involved in them. Items directly acted upon take some of this green quanta charge- Celestial Weight in a nutshell. By adding degrees of separation between the input force and the force affecting the target, one allows the green quanta to leak out or simply not be applied as much. For example, the act of firing a bow would have something close to the 'maximum' quanta given the energy required to draw it (manually throwing the projectile would be even more efficient, in theory). A crossbow using the exact same bow and draw weight would not have as much green quanta, as it would leak out in the time between priming the weapon and pulling the trigger. A theoretical firearm with similar kinetic energy powered purely by external sources such as gunpowder would only have the quanta infused from the act of pulling the trigger- far, far less.
There exists a hypothetical 'zen' point past which the complete lack of intent pushes an action into a vague 'Chaos' alignment, thereby bypassing all Significance-based probability manipulation.
In direct opposition to the soul-force effect, reducing the impact of an action, is Internal Observer Effect (Celestial flubbing) . Without flubbing, the Soul-Force effect would only act as potential plus; one could never go below the expected effect from a purely mundane system. Flubbing reduces the impact of an action such that it can actually be less effective than what conventional physics dictates.

Consequences

Killing intent is more than just a psychological thing to stop one holding back, it inherently charges one's actions with more Significance.
Humans are just overall more durable. Less likely to break things from falls etc.
More injuries are from self-inflicted accidents such as hammering one's finger when missing a nail.
Weapons which use external energy sources such as firearms and focused magical beams are less effective and rarely utilized on an individual level. For a siege weapon, it's probably fine due to sheer power (and not directed at conscious entities anyway), but individual-level conflicts don't respond well to weapons lacking intent.