I would say that Steve Jobs wears the same outfit every time we see him for the same reason Big Bird or Homer Simpson wear the same out fit every time we see them. That is what we are used to seeing them in and we associate the brand of each character with that particular outfit.

I do not see it being any more complex than that.. it is branding. That outfit is his costume, his personal brand, his most recognizable get up, it is his "Look at me I'm Mr. Apple" uniform.

Its simple really, super hero's need costumes. He has come to save us from the legion of doom more commonly known as Microsoft.

really i think he focuses ALOT on his image, and part of this is what clothes you choose to wear. While Steve certainly has a personal style, this might be "unfashionable" and it could change the way people think about Steve and ultimately Apple. Imagine if he was wearing socks with sandals and a Hawaiian floral shirt, people would immediately relate this to his intelligence. Apple is a very fashionable company, if Steve Jobs wasn't fashionable or at least decent looking, this would effect the style of Apple. I think there is a lot more time and energy spent on his outfits and basic image than we would think.

Jobs' outfit combines two elements. First of all, the jeans say that Jobs is not a "suit" and this echoes the iconic Mac commercial where a young woman (wearing the only color in the commercial) defies the uniformity of the crowd and destroys the image of the tyrant.
Second, black creates a psychological, but not a visual contrast. It is "no color" and opaque (something through which nothing is seen). Psychologically, Jobs wants Apple to be opaque--we create great products that just work, and consumers don't need to see how they work, because they work. And don't wonder why a Mac doesn't have a floppy drive or an iPad doesn't have a camera. The internal decisions and structures (a buttoned shirt under a jacket, for example) don't matter, and nothing that doesn't matter is visible (e.g, it is simple and unadorned). This carries through in Apple's general design philosophy.
Another practical reason for black is that it will not interfere visually with any of the colors on the presentation (the same reason that I use a completely black background on my computer when editing photos). Jobs uses extremely simple slides, often simply displaying a single object, and a presenter in black does not interfere with the viewer's focus by creating a moving, colored object just outside the visual angle of the image on the screen.

He's probably one of those people who have several suits that are tailored just right and look the same. Some people are just that focused on their work that they don't want to take the extra time to go shopping. These are people who go shopping once per year. The suit fits right; they have it tailored right and they order a hundred of them! Some men tell me that they don't like shopping and they want to make it as easy and as infrequent as possible. He could afford a designer, especially if he thinks it will help his image and his business. Maybe he will. It might take the drudgery off the shopping experience that some men complain of. Then again, I've never personally met Steve Jobs, not that I remember anyways. I'm just speculating from my experience in talking to men while I shop. And they ask me, "Why am I here? I hate to shop!" And I console them and tell them, "It's not all that bad" as I guide them in the right direction so that it makes a better shopping experience for them! Women and people who like shopping have the maze of shopping malls already figured out, and those who don't have a huge work load to rush back to enjoy it. Maybe he likes playing golf better than shopping in his free time, or maybe if the stress is taken out of the shopping experience, he might like it!

Because he is comfortable in it, he really doesn't worry about fashion and mostly because he can.