They can get the prices wrong. They can stock the wrong inventory. If you purchase an established business, the chances for these errors are minimized. Also minimizing the chances for failure are the established systems of a going business. Sales and marketing, accounting, inventory tracking, employee payroll and production systems are all in place.
Employees know what to do.
Customers even have established patterns of relating to the business. In a start-up, all systems have to be created from scratch. Most people who have started a business from scratch can tell you how difficult this is. Just think, for example, how long it took you to learn and fully utilize your last new software program. There are at least 100 different 'systems' of how things are done in a small business.