You're in the inside lane approaching a sliproad onto the motorway. Several cars are accelerating down the sliproad to join your lane. To make it easier, you indicate to move right and give them space. Idiot in the middle lane sees what you're doing. He's slightly behind but not enough for you to safely pull in front of him? But does he slow down? No, he's either oblivious to the road in front of him, or there's no way he's going to let off the gas ever so slightly, to make enough space for you to pull out.
It takes cooperation to make these roads work.