Because none of the business-side people bother to learn, and all the commercial tools allow them to avoid learning.
User stories that are written solely from the person at the keyboard, with no thought to the different parts of the architecture - so all the estimates require input from UI, middleware, and server-side staff, and can't be subdivided by the tool into group-specific work
Business-types who cannot define a feature with any precision - "customer will get a message with the offer", neglecting all the many ways the system needs to not repeat the message.
