You might be surprised how well you can gauge amperage demand with mechanical demand. I mean it's a linear relationship with respect to voltage so it's pretty easy to scale it. Though you're right, manufacturing tolerances and system state could make it unreliable.

It would be more reliable to use an optical sensor, but the cost of a current gauge is a few lines of code and a sense resistor along with the controller which would already be there for other functions. That's compared to the greater cost of an optical sensor with emitter and receiver.

It's almost always about the cheapest way to do something rather than best. There was a time engineers would do something because it's best even if it cost a little more, but those days are long gone, except maybe for NASA. With design often contracted to cheap labor markets, cost considerations rise well above all else. The new adage is make stuff for half the cost and make the customer buy twice at twice the price.