It's a mob thing. I'm always amazed that, sitting next to the Puget Sound, which is quite large, and very deep in places, with all kinds of boats, ferries, bridges, etc., that you just never hear of anyone allegedly dumping a murder weapon in the water around here. We also have plenty of lakes and other such handy places, yet time after time, the cops come up with a murder weapon.
My favorite was years ago, in the county where I used to work. A guy was alleged to have committed a murder with an axe. You'd think, shouldn't be too hard to get rid of. Hell, you can even burn the handle, just drop the head over the side during a $4 ferry ride, no one would think anything about it.
No, this genius took the axe back to the hardware store. For a refund. He wiped it down, cleaned it as best he could, and returned the friggin' murder weapon for something like a $10-15 refund.
At least they couldn't accuse him of being wasteful, I suppose. I doubt they'd let him take the axe into prison to serve his time.