Because they're comfortable, familiar, and they can get away with it? I think it takes a particularly cynical and angry mind to stretch the choice of clothing to be a tool of oppression on the part of the founders.
It's the same as buying a few pizzas for your employees to get them to code till sunrise instead of paying them proper overtime. Impressionable college grads with little work experience elsewhere will take that kind of behavior for granted, and assume it is the cost of their (measly) equity. Essentially, you want them to be suckers. Comfortable suckers.
Most of these guys telling you what to do, where to live and which technologies or schools that matter are Ivy League alumni multimillionaires. Many from a time before a widespread Internet and grants for poor students. When was the last time you saw any of them sit down with someone who were really challenging their views?
Edit: For somewhat of an example of the last thing there are two episodes of This Week In Startups with DHH.
https://www.youtube.com/watch?v=XDGHxO6N3Ms&t=15m30s https://www.youtube.com/watch?v=jzERXJgi5vQ&t=7m11s
Additionally, after work most of my friends have to change outfits to participate in social events. Business casual dress is useless, now I wear jeans, athletic shorts, and t-shirts to the office.