Think the same reason you buy any working business. The previous owner has managed to solve a great number of non-obvious operational problems. To name a few that come to mind:

* Established supply chain - Working agreements with suppliers (no need to find and negotiate suppliers) - Working logistics. Scheduling of supplies is sorted, you have reliable delivery slots with all supplies

* Employees. No need to find and train labour.

* Working pay roll, accounting, taxes. No need to create that infrastructure from scratch.

* Know good business model, with auditable accounts. You substantially de-risk the investment, because you have meaningful historical data to estimate future performance from.

* Physical infrastructure already setup. No need to find a building, buy equipment (shelves, tills etc) and setup equipment.

* Business generates revenue for new owner immediately after purchase. No time needed to solve all of the above problems, instead time can be used to grow or stabilise business.