Well, technically, software piracy (yarr!) is stealing according to the second meaning of "to steal": "to appropriate (ideas, credit, words, etc.) without right or acknowledgment."
There are however a couple of reasons why it is not stealing, usually:
With the exception of developers who use pirated software for their own private company, the general public uses software only for their own gain (not money). Like a lot of photoshop software is pirated because of the price of the package being too much for poor people who still want to learn to design in it.
This is a sore point, but bear with me on this one.
It only irritates people who buy it, and Crackers can crack it faster than new technology can be devised. Even before it has been released sometimes.
The biggest problem is that a Steam. For people who pay it offers many things (achievement systems, global leaderboards, extra content, automatic updates, etc.) but it can still be played offline and you can have it installed (and running) on multiple PC's at the same time.
But here is a list of why DRM can be a very bad idea:
It's getting ridiculous, honestly. To the point where downloading a DVD/Bluray is better than buying one in the store. Why? Because there are many ways in which a downloaded copy is superior to an official one.
Seriously, this must stop. When a paying customer has more hassle, more wasted time because of a DVD that doesn't work in a laptop or whatever else just to 'fight piracy'.
Offer, like Steam, extras to people who buy. Online things that are not required for the basic experience, but can add to it.
And no, this is not an excuse to put a buggy product on the shelves just because you can patch it later... (a trend that is all to prevalent lately)