Why assembly lines are inflexible?

an assembly line output is high, the cost per unit is very low.

Not all of them are. Basically, the more complex the item you are making is, the more inflexible the assembly line has to be.

Cars and build-to-order computers are good examples of the two extremes of assembly lines. Auto assembly lines are so inflexible you can't even change model years without shutting down the line for many days. Computer assembly lines can be reconfigured in a very short period of time, and they can be just about any shape you want--you could even sit on the floor and make computers.