Because to steal requires you deprive someone of an item or article which they owned.
From Black's Law Dictionary:
Stolen: Acquired, or possessed, as a result of some wrongful or dishonest act or taking, whereby a person willfully obtains or retains possession of property which belongs to another, without or beyond any permission given, and with the intent to deprive the owner of the benefit of ownership (or possession) permanently.
So, by infringing on someone's copyright, you are stealing because:
1) One of the fundamental powers of owning a copyright is the right to distribute or to choose
not to distribute by any legal means. If you obtain copyrighted material without compensating the IP holder, you've done so by not honoring the terms by which he has offered to distribute the IP. IOW, you have deprived the owner with at least one benefit of ownership- the right to sell (or not sell) it to you.
2) You have intentionally obtained & retained possession of the property without/beyond the permission of the owner.
Theft does not include depriving a previous owner of their property when we're talking intellectual property.
and
Not to say it's not legally a crime to infringe copyright but it's not theft under the legal definition as the owner never lost personal property.
In the broadest sense, it does qualify as theft:
From Black's Law Dictionary:
Theft: The taking of property without the owner's consent...
The fraudulent taking of personal property belonging to another, from his possession or from the possession of some person holding the same for him, without his consent, with intent to deprive the owner of the value of the same, and to appropriate it to the use or benefit of the person taking...
...it includes swindling and embezzlement and that generally, one who obtains possession fo property by lawful means and thereafter appropriates the property to the taker's own use is guilty of a "theft"...
...any of the following acts done with intent to deprive the owner permanently of the possession, use, or benefit of his property: (a) Obtaining or exerting unauthorized control over property; or (b) Obtaining by deception control over the property; or (c) Obtaining by threat control over property; or (d) obtaining control over stolen property knowing the property to be stolen by another
Copyright infringement satisfies several clauses above, (a) at the very least (especially that "exerting unauthorized control over" language), and quite often in the age of file sharing, (d).
Knowledge should be free you know. If you say no, then your ethics are questioned, not his.
Innovation flourishes most when those who invest their time, resources, money and efforts into creating IP have the right and ability to defend controlling that IP. Not only that, laws that protect IP create jobs by making it possible to make R&D pay off, and that makes more products available to the consumer...not less.* If Coke couldn't protect their recipe, they never would have been able to manufacture & market it on a mass scale- ditto any company with proprietary IP.
It is not an accident that the USA, Japan and other 1st world capitalist societies have succeeded in tech and various IP dominated fields. It is also not an accident that places where enforcement is lax- like China- are having difficulty in actually producing innovation of their own. Just last year, it was reported that Chinese tech firms are having their own discoveries stolen by homegrown pirates who learned how do do so by stealing from the West, leading to a declining investment in R&D. You don't invest in R&D if you can't reap the rewards, and as a result, your company stagnates...unless you do a little theft yourself.
* It is actually the failure or non-enforcement of
other aspects of the law, like Anti-trust, that removes products from consumer's grasp, not enforcement of IP laws.