Even though this narrative is hugely removed from the realities of startup life (most college startups go nowhere, the most successful founders tend to be in their middle age with significant financial stability, etc), it is still romanticized by founders and startup employees and other people who really should know better. So then why do they buy it?
To get people to work harder for you. Spending every waking moment in front of a customer or a computer screen eating bulk ramen sounds like a great montage scene in the movie you'll have an EP credit in, and this distorted reality is even easier to sell to impressionable young college grads who have maybe .1% of the equity (in options!) that founders and VCs get to keep.
Why are jeans and hoodies the fashion choice of founders? Because if everyone is used to dressing like a poor college student, they tend not to notice how little they're truly being compensated.
This isn't some big founder/VC conspiracy, it's complicit common sense.