The reason you can get away with sneakers and jeans is that how you look doesn't matter, what you have to say and what you do is what matters. Investors aren't giving you money because of your fashion sense, they're giving you money so that they can get money back.
I know this might sound shallow but the first time I meet someone, I actually judge that person based on the way he or she dresses (if I suspect that he could afford dressing properly). It tells me whether they care about themselves, whether they have a good taste. Ultimately yes, in my experience, there's a correlation between "spiffy clothing" and "trustworthiness".
And I'm convinced that most people do the same, whether they admit it or not.
Fashion is clearly very subjective but it is also a visual language. If the way you dress within a context is communicating that you don't care about yourself, you aren't dressed properly.
I wear what is comfortable precisely because I care about myself. By your logic, dressing in any manner other than shorts and t-shirts would either be communicating that I don't care about myself, or would be a lie.
