Actually they don't.

And if they are healthy after a gank they may be coming for you next.

Pan to the gank and watch. See if anyone uses a summoner spell or ultimate.

Check how healthy the enemy junger is and which direction their character model is facing as it goes into the fog of war.

They may stop to do scuttle crab, but if they were facing down river to your lane they may be coming for your next.

Graves, Lee Sin, Kindred, Elise, and now Kai'Sa are the champions that I see do this most frequently because they have quick clears, tend to stay healthy during their clear, and bring a lot of damage with them which makes them good at drive by ganks that may not net anything more than a big chunk of damage to you allowing their laner to all-in you later.

They gank Top, then they gank Mid, then they gank Bot. And almost every time my laners are oblivious to what is happening on top of quite often ignoring pings.

It may be possible to countergank and if you see this pattern developing, ask your jungler to come by your lane for just such a purpose.

Otherwise, try to push out before they get to your lane and drop a ward in the river to spot the gank coming.

Or if you are going to get pushed in then get a ward out on your side that will help spot the jungler trying to flank your current position.