Instacart's founder is fond of saying that Instacart couldn't have existed before 2012. I suspect what he means is that there were a collection of technical & social changes that happened in the early 2010s that let him recast the problem Webvan was solving in an economical way. These were: 1) smartphones allow real-time coordination across thousands of workers, without hiring lots of managers 2) cloud-computing lets you run big-data algorithms to feed those instructions to thousands of workers, without building data centers 3) because of the Great Recession, thousands of workers were unemployed and desperate for some way to earn money 4) increasing urbanization has clustered people together in a city and made them disinclined to drive, which increases the demand for a grocery-shopping service and decreases the cost of servicing them and 5) everybody had Internet access and cellphones.
By contrast, WebVan spent a billion dollars building warehouses, at a time when the total number of Internet users was < 150 million. They bought their own fleet of delivery vans and hired their own drivers, at a time when employment was full and labor costs were high. They had to invest much more capital for a much smaller market, and then had much higher variable costs.
Being a "startup" doesn't repeal the laws of business - you still have to pay for labor, generate returns on capital, generate more value than you charge, and charge more than you spend. But because computers operate millions of times faster than humans and don't require wages, if you setup the business model right you can realize huge efficiencies of scale. "Setup the business model right" is the tricky part - WebVan thought that "Internet ordering" was a crucial part of the business model for online grocery delivery, but it turned out that "use existing supermarket infrastructure" and "coordinate lots of shoppers so they can work very efficiently" were the real keys, and the technology to do that hadn't been invented yet.