I.e, a $10,000 outside "investment" allows you to sell 100, $100 gift cards for $120. Those profits get reinvested on the books, and now you have $12,000 to sell 120, $100 gift cards. Lather, rinse, repeat.
With a car was, 1000 washes, at 5m per wash is 84 hours of active operating time. This creates an upper limit on the amount of money that can reasonably flow through the company, since 84 hours is roughly 3 hours a day of utilization per month. You might be able to get by with about double that many washes without raising suspicion. But all it takes is a peak at the company's water bill to determine how accurate that figure really is.
Plus the operating expenses are much higher, as it requires a specialized building, land, etc. Whereas the online retail requires a computer and some software. It's easier to move and hide. There's just so many benefits to using online retailers over brick and mortar operations.
What kind of a mom and pop store sells $3M year in amazon gift cards?