The subset "Americans who use Instacart for Whole Foods deliveries" combined with the subset "Americans who subscribe to Amazon Fresh" might just add up to 100% of a subset "Americans who pay to have their groceries delivered".
In other words, Amazon picked up the top end of the market, while Instacart is left with slim pickings. While their investor deck might feature millions of people worldwide ordering deliveries from their local grocery place, in reality most people (a) don't consider grocery shopping stressful, (b) prefer to pick their own produce based on personal requirements on ripeness/size/shapes/colors, (c) happen to drive by the grocery store anyways on their way from work or (d) are low-income and would rather not pay the delivery fees.