Comment by Karliss

The main keywords here are high gear ratio and cheap parts. I am not making universal claims about all use cases. For most sane designs, sure electric motors are great at converting energy into torque. Normally you would want most of the electric energy to be turned into useful work. Which implies that electric power directly correlates with mechanical power or torque*speed. The same can't always be said about gearboxes, especially for some of those targeted at hobby/toy use cases with crazy gear ratios like 1000:1 or higher. Gear ratio can be so high that you will strip the gears or make the shaft connection slip long before you can apply sufficient torque to slow down the dc motor which is almost free spinning and spending most of it's energy to overcome friction in first few gearbox stages instead of doing useful work. In a toy or hobby project when you want something to spin slowly it might not matter that the gearbox is <10% efficient if it allows you to reuse same cheap brushed DC motor as hundreds of other toys. Increased torque is partially a side effect of slowing down the tiny motor not the primary goal, although at those slower speeds you probably want slightly higher torque but nowhere near as much as what the gear ratio gives in theory. Even some non toy use cases like cheap home appliances might occasionally use crappy inefficient gearboxes and dumbest electronics possible, especially if the 1-3W motor isn't main consumer of power. There might not even be motor controller or MCU to monitor the current.

It's not always question of lot of work for hobbyist, as it is result of using cheap of the shelf parts and modules which are optimized with different goal in mind and give very poor signal to noise ratio. Doesn't matter how much characterization you do if the change in temperature, grease viscosity and distribution, plastic flex produces higher variance in motor load than any force you can apply to final gearbox stage. I guess the more careful choice of suitable combination of parts from more specialized stores can be considered "lot of work for hobbyist" compared to picking first result on amazon or whatever you found in your junk bin so your argument still stands.

Of course high gear ratio or slow speed doesn't always mean inefficient gearboxes. There are solutions for slow rotation with or without high gear ratio which are reasonably efficient thus allowing to use motor current for estimating torque. And any serious or well designed equipment will use them. But that usually means more complex gearbox, motor controller or purpose built electric motors all of which is either more expensive or require high MOQ orders from manufacturers.