Pain is a message in the brain. It says "this is bad!" so we can act and avoid it. Pain lets us avoid damage. So we live longer. So we have more offspring. So those offspring inherit pain reactions.
Pain only works if you can stop it. If pain cannot be stopped then it distracts us, so we do things badly. So we don't live as long. So we have fewer offspring. So, over many generations, we will only have the kind of pain that we can avoid.
Random nature changes very slowly, so living things can always avoid most pain. Takes a meteor strike for example. Meteor strikes are so rare that, after the initial impact, everything else is normal and largely predictable. To the animals involved, a meteor strike is just like a sudden predator, then a really bad winter, then (for the worst events) like living in a could country where food does not grow. These are all very familiar scenarios to animals, and animals have evolved to cope with them without wasteful pain.
Humans make multiple, constant changes, so we cannot evolve quickly enough. We plough the fields, lay roads, breed animals for farms, build fences, and so on. So life is faced with constant situations where choices do not work. So, constant pain: where the brain says "avoid this!" and there is no way to avoid it.
The obvious solution is to slow right down. Go back to a pre-farming society, where any change takes many generations.
A possible solution is to use our brains to notice when someone (human or animal) is suffering, and find a solution. But we have no incentive for this, other than to avoid cognitive dissonance. Because those with the most power in society are those who benefit the most from society. If they undermine society then they reduce their own power. So they will make token efforts to reassure the victims, while always increasing inequality (which reduces the freedom to escape pain), increasing the number of factory farms (reducing the freedom of animals to escape pain), increasing the rate of technological change, etc.
In the long term, evolution will continue as before. Suffering will end because humans will end. Nature always wins in the end.
Humans know that when a severe injury hits, pain is often not felt until later. This is because, in extreme trauma, pain has no survival benefit (you already know the lion is eating you!) but does have a survival cost (it distracts you just when focussed decision making is most valuable. So trauma victims often say "it was like I was watching it happen to someone else". We see the same in animals, such as when a zebra stops struggling and lies there as the lion eats it.
Humans feel an adrenalin rush during moments of trauma, We might act quickly to help others, just as a bird might scream a warning to others. There is little chance that the others can save the animal, so the scream is a warning, not a cry for help. It is a result of adrenalin, and the animal needs to think clearly. Pain would be a distraction so would be evolved out at that moment (it would return when it served a purpose: hours later when avoiding further damage is possible.)
Not every predator is perfect. Remaining calm (not blinded by pain) sometimes tricks the predator into relaxing its grip to get a better bite, and at that moment the prey can kick and run, and some will have later offspring. More importantly, genes are measured at the group level. As you are being eaten, you can concentrate on warning your family. Or you can become calm, encouraging the lions to relax, knowing a meal is guaranteed, and this gives your family vital seconds to escape.
Severe arthritis is caused by humans changing faster than evolution can catch up. Our ancestors seldom lived past their sixties, so arthritis was less common. And they were nomads: if they could not walk then they would have to curl up in some corner and go to sleep forever: resulting in a week or so of pain at the very most. The same would be true of all fatal illnesses: you are either highly functional or you are dead quickly. We see the same in animals that evolve naturally. So prolonged extreme pain is an invention of modern humans and our absurd desire to put quantity of life before quality. Oh, and one more thing: our ancestors did not have politicians saying "do not chew cannabis plants" (or whatever plants released pain: hunter gatherers are experts on all plants.)