It's about presenting a put-together and professional image. If you want clients/co-workers to take you seriously and trust that you know what you are doing, you have to look and conduct yourself like a professional. It is also about fitting in with a company's culture. You don't have to look the same or behave the same as others, but you do have to complement the "vibe" so to speak. They want to hire someone who is going to be a "team player." That doesn't mean can't be yourself--you just have to be a professional version of yourself. It's a lot like using appropriate language at work (no swearing, no rudeness, etc), or knowing how to behave appropriately with co-workers/clients. You don't have to look "fancy," you just have to look like you are "at work" and not on your living room couch or at the beach. You could get away with black jeans and a polo shirt, black jeans and a blazer, khakis and a button-up, khakis and a loose sweater. There are a million options for looking professional without dressing femme or fancy or being uncomfortable. Other posters have suggested so many great options, but you don't seem to like any of them. It seems like you are resistant to the whole "idea" of looking professional, rather than just the clothes themselves. It's not that you can't work on the computer in sweats & a tee-- it's that your co-workers/clients will be less likely to trust your advice or want your help on their computers, others walking around the office might wonder if you are supposed to be there, and you will call attention to yourself if you look significantly underdressed in comparison to others. I'm not saying this is necessarily fair, but it is the way things work. I'm a professor and I'm young-ish but look even younger. I significantly over-dress in comparison to many of my colleagues because, if I don't, my students don't take me as seriously or treat me with as much respect. It's not "fair"-- but I dress that way so that the attention is on my lectures instead of on "me." The more professional I come across, the smoother the class runs.