Answer by Alex Lightman:
Because having more money makes you more of whatever it is that are already are.
Because of one of the reasons to work hard and to be successful is to be able to dress the way you want, speak the way you want, and act the way you want, with the people you want, in the places you want.
Because VCs fund a company based on the track record of the entrepreneurial team, the potential of the venture, and the pitch, and people are typically on their best behavior for that hour or so.
If I may be forgiven for thinking that you are not a native speaker of English and from outside America, it's very possible that you are from a country whose culture is more formal and rigid than that of the US in general, and Silicon Valley, in particular.
Nolan Bushnell started the Silicon Valley trend of casual Fridays, which ended up changing formal dress. This was 40 years ago, so it's not surprising that trend has been to greater laxness.
There is a silver lining: if you are polite, well groomed and well dressed, you might have a small temporary competitive advantage in gaining the benefit of the doubt, until a customer can determine which offering at which price is preferable.
That said, you have not mentioned the most bizarre behavior, such as wearing pajamas to a board meeting of SpaceX.
Sorry, but even I cannot explain or justify that. It's utterly mysterious.
Perhaps it's a dramatic omen about being in cryosleep for six months before getting to Mars, and symbolizes a spaceship filled with people whose culture revolves around around sleep…