OK, so you're saying you don't need to have rainy day funds at the start of a new business, but should build them up over time, right? So, that leads to two potential situations. Let's use the restaurant example.

First possibility is the one most restaurants face. If they're lucky and the business is actually viable, they will have a profit margin of 3-5%. For a single location restaurant, that profit will be enough for the proprietor to make a decent living hopefully, but not a whole lot more. Should be enough for them to save up a few months of personal expenses sure. But saving up enough cash to pay rent, suppliers, and staff, for months, with no income... would take forever with those margins. It's just not possible for most businesses of that sort.

But let's also look at the other possibility. Say you've got a real hit restaurant (or another type of business with larger margins) and your margins aren't 3-5, but 15-20%. Now you're saving real money. So yes, you could sock it away under the proverbial mattress. But if you're that successful, it means people really value the product you're providing. Instead of hoarding your profits in case of a pandemic, you could instead invest those profits into opening a second location, allowing twice as many people to benefit from the service you're providing. In the 99% of years when there isn't a black swan event, that option is better not only for the business owner, but for society as a whole. So even for very successful businesses that could afford to keep a large amount of cash around, we're usually better off if they spend that money to do more of what's making them successful in the first place.

Of course occasionally there will indeed be a pandemic, at which point government can help out to prevent both types of businesses from failing, so that when the event passes they can go back to generating value.