Why do some people refuse to wear their seat belt while being aware of the risks? I have multiple friends that simply refuse to wear their seat belt whenever they ride in a car. I make them wear it when in my car, but in any other situation they choose to go without it. They are all aware of the risks of not wearing their seat belt and the implications of even the smallest car accident, so a lack of knowledge is not the problem. Also, I consider them all quite intelligent people, so the ignorance of such a simple task that can be life saving blows my mind.

I cannot see it a denial of evidence because they agree with my side of reasoning, and my question is not specific to my friends, so I am asking why would anyone who knows all the facts still proceed without a seat belt?
I think this is a rather difficult question to answer. Psychology Today sums up some interesting reasons why people totally aware of the risks involved in not wearing a seat belt (or in smoking tobacco products, alcohol abuse, dangerous driving...), still choose not to wear one (light up another cigarette etc.):


Justification of risky behavior may be re-enforced by unreliable anecdotes that "show" how safe it is. For example, a friend or relative never wears a seat belt and smokes every day. People can use this as 'evidence' that not buckling up or smoking are perfectly safe. The dreaded remark comes to mind that "My grampa smokes three packs a day and is in his nineties - now ciggies can't be that bad...". 
In that same line, personal anecdotes can reinforce risky behavior. If it hasn't happened yet, it will never happen. When people have never worn a seat belt for the last five years, they may start assuming that it is safe, because nothing has happened. They believe in the pattern, rather than considering that that luck may well be running out. I never use a seat belt may be used as an argument to conclude that the absence of a bad outcome is evidence that one is somehow 'immune' to the danger involved.
The small statistical chances involved may be used consciously or subconsciously as excuse. When we think about taking another smoke, overeating, drinking yet another beer, or not wearing a seat-belt we often think There's very little chance that the next time it will kill me. This is all too true. The next cigarette may not, and in fact very likely will not, kill you. But what if this is your continual belief for the next 350,000 cigarettes during your lifetime? It may not be that the next cigarette will kill you, but it may be true that holding onto this belief will kill you, eventually.


All of these risks involve a sort of "near-sightedness". We focus on the next one-not on the long string of behaviors. We tend to think that the "next time I go out driving without my seat-belt, there is a very low probability that I will get killed". This is true, but each time increases the probability. 

The underlying problem is that our primitive brains battles itself over short-term rewards versus long-term goals. As a regular quitter of smoking (I've stopped so, so many times) I know this more than anyone. The short-term reward is more important than the long run. Impulsive choices or preferences for short-term rewards result from the emotion-related parts of the brain winning out over the abstract-reasoning parts. Psychological factors other than pure reasoning often drive people's decisions, for example the small nuisance of the seat belt or the dangers of long-term smoking loose over the improved comfort of driving without a seat belt and activating the dopamine release in our limbic system when smoking a ciggie. 

Our evolutionary more ancient emotional limbic brain has a hard time imagining the future, even though our logical, evolutionary more recent neocortical brain (Fig. 1) clearly sees the future consequences of our current actions. Our emotional brain wants to max out that credit card, order dessert and smoke a cigarette. Our logical brain knows we should save for retirement, go for a jog and quit smoking. Quite apparently, the primitive emotional brain often tips the balance and defeats the newer evolved logical brain.


Fig. 1. The three brain areas. source: Big Commerce