"I think it all comes down to people wanting to be like the "business" people they see on TV. The persona of the rigid professional suit-wearing business person is attractive to a lot of people."

No, it comes down to protecting a brand. If you're doing a startup then it's ok to show up to meetings blazed, because your job is just to get work done and if you can still get work done while high then that's fine. But at big companies your job isn't to get work done, because the most valuable asset of a big company isn't the work that gets done but rather the brand. How does this tie into suits? Well, suits are sort of the crescent wrench of attire. If you are a startup then you only have to dress in a way that pleases the niche you're serving. But if you are a big business then by definition you are interacting with people from multiple niches all day long. Everyone is comfortable dealing with someone wearing a suit, whereas if you wear surfing clothes to work then that only works if you trying to send a message to a small niche of surfers.

The vast majority of the YC kids aren't starting out with a personal brand, so it doesn't matter what they do as long as they get work done. In fact, screwing around will help them build a brand as long as they are only dealing with people within their community, because unique culture is a purple cow that helps you stand out. Luckily most of the YC companies make products where even when they get more marketshare they won't have to act much differently because of quirks related to the high-leverage of software, but most big companies eventually do adopt a different culture as the ways they create value as businesses becomes more nuanced.