The primary reason why most toaster ovens do not accommodate a standard 9" x 13" baking pan is the size limitation of the toaster oven itself. Toaster ovens are designed to be compact and space-saving alternatives to traditional ovens, making them suitable for small kitchens, dorm rooms, or for people with limited countertop space.
A standard 9" x 13" baking pan is relatively large and typically too big to fit inside a standard toaster oven. If manufacturers were to increase the interior size of the toaster oven to accommodate such a large baking pan, it would likely compromise the compact nature of the appliance, defeating the purpose of having a toaster oven in the first place.
However, some larger and more advanced toaster ovens on the market might come with features that allow you to fit a 9" x 13" baking pan or handle slightly larger cookware. These toaster ovens may have removable racks or adjustable rack positions, which provide more flexibility for different pan sizes. But even in these cases, the interior size is still unlikely to be as spacious as a conventional oven.
For baking dishes of that size, a traditional oven or a countertop convection oven would be a more suitable option, as they are designed to handle larger cookware and more substantial cooking tasks. Convection ovens, in particular, can offer a compact yet more spacious alternative to conventional ovens and can handle larger baking pans with ease.