Its a lot of nuance and personal preference really, and without a lot of examples its hard to prove one way or another. But it does have a kind of origin and explanation. I’ll also add here I’m mostly talking about why we call taking of land in war or other conflicts with Native American people theft and not conquest, and I’m not talking as much about obvious theft or treaty violations and so on. This is about the contested term. I'm also mainly focusing on the European perspective.
The many European nations in the age of exploration and beyond sometimes operated under an informal, but important, set of rules when it came to war. This set of informal rules is usually referred to as "just war theory" or jus ad bellum. A just war required a justification for war, called a casus belli, a Latin phrase referring to what provoked the war. Provoke is the key word, war should be thrust upon you. A just war required a justification, and that justification couldn't just be "because I want to," (though many wars were ultimately started by that), you at least had to make up a reason. A made up reason is often called a false flag, and they hold up to scrutiny to varying degrees of success. Winning helps, but isn't the ultimate decider.
Ultimately, a war was supposed to be fought for a good reason. Maybe you were attacked first, so you are justified to exact revenge or retribution up to and including conquering some land from your enemy, or perhaps economically and politically crippling them and overthrowing their leaders. Even if you have justification for war, the result of the war should be relatively proportional. We still operate under these considerations. Even Hitler and Stalin had to make up reasons to invade Poland, however flimsy they were. And disproportionate retribution is still looked down upon, like the punishment inflicted on France after the Franco-Prussian war. The Prussian alliance may have had a good justification for war, but they couldn't impose any terms they desired without reason. I want to be clear I'm not making any personal value judgement here, just commenting on how people felt.
So when someone talks about the United States or other colonial nations "stealing" land from Native American people what they are saying, probably without understanding why or the history behind just war theory, is that what these European people did to Native American people was not just. Most people don't have a problem with other Native American people conquering and displacing each other - they either had a justification for war or operated under their own theory of warfare outside European thought. But when such an overwhelming force pushes Native American nations aside and takes their land just because they want it, it looks like theft. So people call it theft. In many cases there was no formal declaration of war, or if there was the retribution was disproportionate. Annexing an entire nation and displacing its people is a huge deal, there is a reason it didn't happen much after the mid 1600's in Europe. It went against just war theory. And when it did happen it pissed off everyone.
There are some wars between European descended people and Native Americans that seem to fall under just warfare theory. The newly formed United States of America took land from the Native American tribes who allied with the British Empire. And after the American Civil War the United States removed many of the Native American people who fought for the Confederacy. In these cases the Native American people, despite their own reasons for their actions, cast their lot in with an enemy in a just war and risked everything. Now, whether the consequences for losing were fair and justified is another matter. Many people seem to think the consequences were disproportionate. After all, the Confederate people who started the war and did 99% of the fighting were not stripped of their property and removed to Oklahoma and so on. It was just the Native Americans, so even though the war may have been just the consequences don't feel just, or feel like conquest. They feel like theft at worst or disproportionate at best. The Mexican-American war was sometimes referred to as theft because of the disproportionate land-grab that took place. U.S. Grant said "I was bitterly opposed to [the annexation of Mexico], and to this day, regard the war, which resulted, as one of the most unjust ever waged by a stronger against a weaker nation. It was an instance of a republic following the bad example of European monarchies, in not considering justice in their desire to acquire additional territory." Emphasis mine.
But outside these wars a lot of the interaction between Native Americans and Europeans and their colonies came down to colonists taking land from Native Americans simply because they wanted it and it was there. Pretty theft-y feeling to me. There was no other justification for taking the land, so it looks like theft, not conquest. And most of the justifications for the taking are paper thin and were just a pretext for what they were going to do anyway. Even if they did take it by force or if there was a war, like the Seminole Wars, it lacks the feeling of conquest and the consequences are still disproportionate. In a number of cases the law of the United States said that taking land from certain Native American people was illegal, but the states or the army did it anyway. So people often call it theft. Whether or not it is actually theft is a legal question I suppose. Even "justified" conquest is probably legal theft, its just theft we feel ok with.
Its also worth adding that our own ideas on what is just or not has changed since that land was taken, and to get a full understanding it is worth reading more.
For a bit of historic reading on war theory I recommend:
Carl von Clauswitz: On War
Francis Lieber: Code for the Government of Armies in the Field
Dwight Eisenhower: On the War
From some more modern academic authors:
Michael Walzer: Just and Unjust Wars
Richard Norman: Ethics, Killing, and War
Elizabeth Anscombe article "War and Murder"
I will add that post-9/11 thought on ethics in warfare has changed rapidly and is beyond my reading.
Because land was in fact stolen from Native American tribes as part of the colonizer government's genocide (or genocides) of native tribes.
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