Because this is how you get good people. If you want to offer them what they could get by working someplace else, they may as well work someplace else (likely without the hours, stress, or potential instability of a startup).
Startups offer a lot of non-financial benefits over working at the big established companies.
1) Looser culture. Less likely to have a dress code or an attendance policy.
2) Less meetings, more coding. For someone that wants to get stuff done, way more to do in a startup.
3) Sense of purpose. A huge part of life satisfaction is doing something that actually matters. WAY easier to do this when you are shipping a product to actual customers vs sitting between 6 layers of management at google writing code to improve ad conversion by 0.1%
My bank doesn't accept "sense of purpose" as payment. My real job is for my mortgage and retirement.
An extra $10k+ per year counts a lot when compounded. $10k a year for 30 years, 7% average long term stock market return gets you $1 million.
That could a better life style, retire many years earlier...all without the risk of a Scrappy startup hoping for a unicorn buyout.
Play with the numbers...but that's what you are giving up because you want to wear a graphic t-shirt to work.
Truly, and it's changed my life for the better.
I'm unclear, however, on the relevance of this statement to the extant startup scene -- ref: the latest YC class, for example.