Where have you heard that combat wasn't common?

We shouldn't confuse cliches for history. And in fact the cliche you refer to is limited in scope. Many historians have long held large scale pitched battles to be rare in medieval warfare, but this holding can be misleading in the extreme. The medieval era was a very violent one, one at least as violent as that which preceded and followed it.

It is true pitched battles were likely rare. Logistical and hygienic limitations in an era of decentralized government, limited food markets, low training, and haphazard road infrastructure severely limited the size and permanence of medieval armies. The dearth is somewhat overblown. Obviously some military forces retained professional staffs and longevity, but it is to be noted that financial records seem to show no European nation ever levied armies on par with imperial Rome until the late sixteenth and seventeenth centuries. Such limited size could prevent medieval armies from having the capability to conquer fortified areas quickly or at all.

Pitched battles require several factors. First, your general must be motivated to essentially gamble their fortunes on a single day. Second, his opponents must be similarly enthralled with risk. Third, both armies have to be able to meet on some field to battle. Subpart to all three, there must not be available numerous defensive works that give a defending power relatively cheap protection. Few of these requirements coincided during the medieval era, and despite some /r/trebuchet claims, castles remained very powerful defenses. If a weaker side could simply fall back behind their castles, they had no reason to give battle outside castle walls.

Skirmishes and sieges ate up plenty of lives. Castles and town walls could limit the efficacy of offensive war and provide shelter for an inferior army, but they could not protect a nation entirely. Despite the image of chivalry and moronic tactical aggression, Medieval warfare was a relatively subtle dance of midnight raids, targeted terrorism, and small scale combat between raiding parties. To give one example, outside of the Holy Land, Richard the Lionheart likely never fought a per se battle, but he skirmished with the Marshal, the French, and his own lords a hundred times in siege and in raid. He and his knights were no strangers to violence. Additionally, check on any parish roll. Violent crime and raiding were endemic, especially under weak kings or in borderland Marches.

But even if your claim were true and combat was rare, there is no reason why professional soldiers would not invest in body armor appropriate to the times. The Roman legions didn't throw away their swords and turn them into plowshares during the relatively peaceful Principate. No European army has fully demobilized despite one of the most peaceful eras in history. Soldiers like protection, even when they're barely soldiering. And medieval soldiers likely fought a lot.

So, question. There have been very, very few pitched battles fought in Afghanistan during the US/NATO intervention. For the most part, that conflict has been marked by low-level fighting between small units. Yet NATO soldiers employ not only a wide range of weapons, but body armor and even armored vehicles. Why? Because low-level warfare is not the same as warfare without combat. Arguably, a war of platoons and companies rather than divisions and corps makes individual equipment more important. If my twenty guys run into your twenty guys, I really want my twenty guys to be better equipped than yours.

I disagree with /u/DavidlikesPeace in that I think pitched battles were quite rare. That is because medieval warfare was fundamentally different than the style of warfare we are accustomed to. From petty lords to kings, medievals did not conceive of warfare in Clausewitzian terms. Their purpose for making war was generally not to destroy their opponents' ability to resist. If what you want out of a war is to secure grazing rights, or to retake a disputed piece of land, or even to secure a claim, you can do that without the expense and risk of fighting huge battles. It's much more cost effective to wear down an opponent via raiding and, if necessary, sieges, until he gives in and accedes to your demands. If a raiding party gets gobbled up, you can generally recover from that; if a major army is crushed, that's something you're going to have trouble coming back from.

However, I do agree with /u/DavidlikesPeace that this should not be taken to imply that medieval warfare was in any sense bloodless. Lords and kings were not at all eager to let their lands be ravaged. Counter-raids and skirmishes between bands of men - often cavalry - were a normal and expected part of warfare. These encounters could be quite bloody, even if they only involved dozens or scores of men. I'm going to quote from Froissart to illustrate this better.

Whilst the French and Bretons were considering the most advantageous manner to begin the onset, sir John Chandos arrived with his company, his banner displayed and flying in the wind. This was borne by a valiant man at arms, called James Allen, and was a pile gules on a field argent. They might be about forty lances, who eagerly hastened to meet the French. As the English arrived at a small hillock, about three furlongs from the bridge, the French servants, who were between this hillock and the bridge, saw them, and being much frightened, said, "Come away: let us save ourselves and our horses." They therefore ran off, leaving their masters to shift as well as they could.

When sir John Chandos, with displayed banner, was come up to the French, whom he thought very lightly of, he began from horseback to rail at them, saying, "Do you hear, Frenchmen! you are mischievous men at arms: you make incursions night and day at your pleasure: you take towns and castles in Poitou, of which I am seneschal. You ransom poor people without my leave, as if the country were your own; but, by God, it is not. Sir Louis, sir Louis, you and Carnet are too much the masters. It is upwards of a year and a half that I have been endeavouring to meet you. Now, thanks to God, I do so, and will tell you my mind. We will now try which of us is the strongest in this country. It has often been told me, that you were very desirous of seeing me: you have now that pleasure. I am John Chandos; look at me well; and, if God please, we will now put to the proof your great deeds of arms which are so renowned." With such words as these did sir John Chandos greet them: he would not have wished to have been anywhere else, so eager was he to fight with them.

Sir Louis and Carnet kept themselves in a close body, as if they were willing to engage. Lord Thomas Percy and the English on the other side of the bridge knew nothing of what had passed, for the bridge was very high in the middle, which prevented them from seeing over it.

During this scoffing of sir John Chandos, a Breton drew his sword, and could not resist from beginning the battle: he struck an English squire, named Simkin Dodenhale, and beat him so much about the breast with his sword that he knocked him off his horse on the ground. Sir John Chandos, who heard the noise behind him, turned round, and saw his squire on the ground and persons beating him. This enraged him more than before: he said to his men, "Sirs, what are you about? how suffer you this man to be slain? Dismount, dismount:" and at the instant he was on foot, as were all his company. Simkin was rescued, and the battle began.

Sir John Chandos, who was a strong and bold knight, and cool in all his undertakings, had his banner advanced before him, surrounded by his men, with the scutcheon above his arms. He himself was dressed in a large robe which fell to the ground, blazoned with his arms on white sarcenet, argent, a pile gules; one on his breast, and the other on his back; so that he appeared resolved on some adventurous undertaking; and in this state, with sword in hand, he advanced on foot towards the enemy.

This morning there had been hoar-frost, which had made the ground slippery; so that as he marched he entangled his legs with his robe, which was of the longest, and made a stumble: during which time a squire, called James de St. Martin (a strong expert man), made a thrust at him with his lance, which hit him in the face, below the eye, between the nose and forehead. Sir John Chandos did not see the aim of the stroke, for he had lost the eye on that side five years ago, on the heaths of Bordeaux, at the chase of a stag: what added to this misfortune, sir John had not put down his vizor, so that in stumbling he bore upon the lance, and helped it to enter into him. The lance, which had bee struck from a strong arm, hit him so severely that it entered as far as the brain, and then the squire drew it back to him again.

The great pain was too much for sir John, so he fell to the ground, and turned twice over in great agony, like one who had received his death-wound. Indeed, since the blow, he never uttered a word. His people, on seeing this mishap, were like madmen. His uncle, sir Edward Clifford, hastily advanced, and striding over the body (for the French were endeavoring to get possession of it,) defended it most valiantly, and gave such well-directed blows from his sword that none dared to approach him. Two other knights, namely sir John Chambo and sir Bertrand de Cassilies, were like men distracted at seeing their master lie thus on the ground.

The Bretons, who were more numerous than the English, were much rejoiced when they saw their chief thus prostrate, and greatly hoped he was mortally wounded. They therefore advanced, crying out "By God, my lords of England, you will all stay with us, for you cannot now escape."

Where did the attrition come from? Dysentery?