[US] for murderers why wouldn’t they always throw the murder weapon into a river asap?

You get rid of the evidence and it all degrades pretty quick. Most of the time the police won’t be able to dredge up a whole river or large swaths of river near bridges. So why don’t they just throw the gun into a river?

I’m mainly talking about the Luigi case, wouldn’t it make the most sense to just toss the gun in a river and go on about your life? Everything else seemed pretty damn calculated, so why not toss the gun?