I think jungle ganks should have diminuitive results when targeting the same lane the entire game

They do. Kill gold goes down as you repeatedly murder people. Assuming you meant diminishing

they actually do; eventually you'll only be worth 50 gold.

As people mentioned, the victim is worth less and less gold.

Plus at some point you're beating up a dead horse and should probably be focusing on other lanes.

^ What they said.

Also, getting camped lets the other lanes play super aggressive, take neutral objectives without an chance of them contesting, etc. Especially if the lane getting camped doesn't die too much.