Firstly, not everyone who creates a page/layout is necessarily a graphic designer. Many people who create such documents will just use some clip art. Basically it's copy and paste, scale and re-use. IMHO none of the examples you have shown are particularly good examples of graphic design. So, why do people use them? Because they can, and they're cheap, often free. You may call them lazy, but it might in fact just be down to pragmatism. There may also be budget constraints. Somebody has to do work to create custom graphics, and somebody has to pay for that work.
As far as graphic designers are concerned, things such as functional/realistic gears don't necessarily work so well in graphic design. There's a degree of stylisation utilised in almost all good graphic design for things such as logos or icons. It doesn't really matter what the subject is - gears, or other kinds of illustration. It's not really about functionality/realism, rather about aesthetics and what looks good given the constraints of various viewing conditions or use cases.
Here are some examples below demonstrating some of the problems. This shows why simplification/stylisation is often preferable to something realistic/complex, especially problematic at smaller sizes. Graphics are often designed to work at larger scales as well as smaller scales, in a variety of colours, from simple line graphics to filled/coloured shapes, and so graphic designers generally have to be wary of overly complex graphics which will just end up looking like fuzzy/indistinguishable blobs at very small sizes.
Click to view larger