Streaming sites and shopping carts are used by millions of people, but casually after work, maybe twice a day.

LoBs are used intensively for hours on end in a busy work day, and most people's livelihoods depend on them either directly or indirectly. You grow into them. Muscle memory used to be an actual UI design goal, back when TUI's were the most common LoB interface. Not so much anymore when everything is web app and you click through the horrid UI slowly with your mouse pointer, and repeat this identical task 200 times in a day.

edit: We naturally develop a muscle memory even for the worst LoB. But they are not designed with this in mind. If they were, they wouldn't move that one critical pixel around in a mouse driven app.