Well there are 2 likely issues:
They have never thought about the issue at all. Thing is the gears depicted wouldn't work anyway. I mean often graphic designers draw gears that are not even trapezoidal, those can not work on different teeth counts.
But in your example picture looks more like a sprocket than a gear, which is relatable because sprockets are more commonly visible as they are commonly visible in bikes. So in fact its not a gear at all.
They are not aiming for likness as such but ease of reading. So it does not really have to work like a gear.
But realistically most people dont know how gears work. Many times you we a animated gear train with 3 gears. Well yes while idler gears are a thing thats not exactly how gear trains work.
This applies to art in general whenever you see thechnology depicted in art (except mecha anime) its usually aiming to look like plausible technology. Not be plausible technology. Same applies to a lot of other stuff too. Like like flowers and foilage for example. But this is also why AI systems are so successful, most people wouldnt notice the details.
PS: ive spent years on thinking about this, partly because its my job to take art and turn it into physical technology.