It's not that they're specifically throwing coins at the engine, but that the superstition is to chuck some coins at a thing to try and get some luck on your side. Like a mix of wishing well and good luck charm.
The engine is probably because it's accessible, and they're unlikely to miss when hitting it. You also just don't hear stories from people who do chuck a coin at the wing, or into the cabin, because it doesn't cause any problems, so doesn't make news.
I might ask a dumb question as well... But how does a passenger even accomplish that? From inside the plane? Just before boarding? On a high tower throwing coins downward on a low flying plane? Wutttt
Having had a dirt poor illiterate farmer grandmother, I can totally see how some people with the best of intentions might make presumptions about things which sound really stupid for most people.
I mean, at some point when I was a kid and she was staying with us, my grandmother got really confused when she saw the same actor in multiple soap operas because she thought soap operas were real and we had to explain to her the concept of theatre acting.
Ultimatelly it boils down to that person having or not the kind of personality which recognizes their own ignorance on a subject and refrains from acting on such presumptions, and clearly in this case somebody ignorantly presumed it would be a good thing and went ahead and did it.
It's for luck. Like throwing coins into a well or on train tracks. No one said it was a good idea. But it makes some folks feel more calm about their journey, despite wrecking havoc on a turbofan.
I’ve been on planes with groups of people who obviously have never flown before and were getting up and walking around during take off. This does not surprise me at all.