Because while the FOD might not be an immediate problem, it could bounce around and get caught in a mechanical linkage causing a fatal crash, could be rubbing against wires causing a fatal crash, could start on fire (if it has lithium battery) causing a fatal crash, could cause a huge delay when maintenance finds a random part bouncing around later on and they don't know what it's from... you get the idea.
I thought the question is not why this is a problem, but rather why is it possible at all for a loose object to end up in the depths of the AC to cause problems. Is there some reason we cannot avoid this?
From a laymans perspective I think of a car, where dropping something small while driving is unlikely to cause problems in the machinery.
Cars have a lot of trim/seals/insulation/carpeting to reduce road noise and be aesthetically pleasing, and military aircraft cockpits don't care about either of those things, and are largely just a metal tube with racks on which all of the avionics and other cockpit equipment are mounted, with holes in a handful of places where wiring harnesses enter/exit the cockpit. All of that equipment is regularly worked on, removed/replaced, and so it is necessary that it be (relatively) easy to access and remove.
The equivalent would be like if you had to pull all of the instruments, electronics, and seats out of your car every 1000 miles, clean them up, replace faulty bits, and then put it all back again. All of the fancy trim, carpeting, etc., just makes that job harder, so you would probably want a car that doesn't have any of that, and is designed to make doing that kind of work easier, better still if you can avoid having to remove everything, and only have to remove the bits individually that need to be maintained. The down side of course, is that without all of the fancy trim and stuff, there would be gaps where things could fall and be hard to reach, and holes where wiring travels to the engine compartment/trunk/etc. Of course, FOD presents way less of a danger in a car than it does an aircraft, so you might not care if you drop something there, but aside from that, I think the analogy holds up.
Depending on your definition of “small” this is not true. I used to slide my flip flops off while driving until one got wedged under the break pedal and prevented me from breaking.
