For the purposes of this answer, I'm going to call it "stealing" when an individual or small group uses force or stealth or trickery to take possession of of something against the will of the person who rightly possesses it. However, this is not necessarily a faithful translation of the Orcs' language, because you'd expect the morality/lawfulness of an act to be considered important enough to be reflected in the language used to describe it. They'd quite possibly have different words for stealing they consider moral and stealing they consider immoral.
Stealing is immoral amongst humans because we consider property rights to be important. Where society and/or the state regulates property, with the general assent of members of that society, then it is widely considered immoral to run counter to that regulation and redistribute property to yourself. "Stealing" is an offence against the widely-held belief that property rights are needed, against the authority of the state to administer those rights, and against the authority of individuals over their property.
Taxation is an interesting example, when thinking how different cultures might treat property differently, because those who disagree with it do consider it a form of theft. Those who consider taxation to be just, do so on the basis that the believe the right of the state to tax has sufficient foundation to override property rights, despite the disagreement of those who say the state cannot have rights that override individual rights. So a Libertarian views our culture as one in which a particular form of theft is considered right. But it's not what I'm calling "stealing" here, solely in that it's perpetrated by the near-consensus of society as a whole, not just by some individual who fancies a new TV.
I'm not a Libertarian, and I don't consider property to be absolutely fundamental. However, it is clearly rather important to the way our society is constructed, and also to historical societies going back thousands of years. So it's no small change to have everyone running around trying to take stuff from each other all the time. For society to support this, you'd need something like:
- No concept of "ownership", only of "possession". So this is the stick I'm holding. If you took it then it would be the stick you're holding. I have no idea whether I'll be holding a stick or not by tomorrow.
- Ownership is somewhat acknowledged, but the responsibility of society as a whole to actively support it is not. This is my stick, and it's nobody's business but mine to ensure that my stick stays with me. I'll be personally inconvenienced if someone takes my stick away, but not going to think the person who has it is immoral, or has failed in their responsibility to respect my stick, merely that they've got the better of me.
- Ownership is acknowledged, but the right to own more than you can defend is not. This is my stick, and ordinarily it would be wrong for you to take it, but the warehouse full of my sticks just down the road puts a rather different complexion on things.
In all these cases, your Orcish society will lack the consequences of the ability we have in our society to maintain a warehouse full of sticks (perhaps as part of the role of a stick manufacturer or trader), and most importantly to share the burden of defending that warehouse across society as a whole by taking action collectively against stick-raiders and by personally refraining from taking sticks even when we have the opportunity. The difference between an immoral act and mere business, is that people respond negatively to immoral acts that would otherwise be nothing to do with them, and refrain from immoral acts that would ostensibly benefit them.
Even in pre-modern societies that didn't have police (in fact, especially in such societies), the group as a whole was still upholding property rights by condemning thieves. With no such condemnation, property rights either don't exist at all or else are much more difficult to uphold, and therefore the things we reward with great wealth in our society (crucially: trade and organisation of labour) are less well-rewarded. There aren't going to be a whole lot of Orc merchants, because being a merchant is so much more difficult if the defence of your stuff is not "socialised" to any extent at all. By "socialised" I don't necessarily mean there needs to be a police force which everyone helps pay for, merely that people on the whole will go out of their way to assist a wronged party and hinder an immoral actor, simply because they recognise the act as immoral. If it's not immoral, you don't get that.
I'm not a ruthless free-marketeer, and I don't think like Libertarians do that property is the ultimate right. I don't think merchants have to be able to keep all their profit in order to be incentivised enough to make the whole thing worthwhile. But they have to get something, and public recognition of their entitlement to whatever's left after tax is something.
Merchants in a medieval-style society expect to hire guards, of course, and rich people in the modern world hire guards too. But when those guards catch a thief, they rely on the support of society to do something about it. So when you hire a guard you get more for you money than just physically obstructing thieves, you also get that thieves the guards catch are "dealt with" and thieves who fear being "dealt with" by society are deterred. There's a cost of doing business, but not so high as cost as it might be. You might think that private guards could just beat the thief up or kill them on the spot, so that thieves are "dealt with" without the help of morality. But if stealing isn't considered wrong, and killing people is considered wrong, then actually you can't do that because then you're the bad guy and society acts against you for beating people up who've done nothing wrong. There's a direct tangible benefit to property owners in society considering theft wrong, even if it doesn't look like society is really doing much about that belief.
However, you don't in practice get large-scale trade without large-scale rewards, and if the prevalence of theft reaches the extent that it's impractical to hold surplus, and actually there's no reward at all in accumulating wealth by trade or management, then economic growth and the large-scale co-ordination of resources may be very difficult to achieve. Furthermore, if any surplus is subject to being taken, then only those physically powerful enough to hold onto a surplus is going to have one, and everyone else will live basically hand to mouth.
Farming might be supported in what you describe: you don't raid someone's small field that they need to live, and you don't take their only coat, because that's too close to killing them. But as soon as someone has two fields, or a slightly nicer coat that they wear on special occasions? Then it's in someone else's interest to stop farming and instead make a living taking that person's surplus. If excess production isn't somehow rewarded then nobody will have anything nice. Of all the ways that have been attempted to reward the generation of surplus, property so far is the least useless.