Because there is a market for cheap laptop-form factor devices, and it is also the reason why schools choose Chromebooks over $349 iPad.I love how people can come to the conclusion it'll run iPadOS.
If Apple wanted something 13-inch to run iPadOS, they would just build a bigger version of the $349 iPad. Why would Apple take the convoluted path of making a low-cost MacBook to run iPadOS?