Agreed, drivers often do not check for this. If I'm merging into a lane, I prefer to do so when not sliding next to another car. If it's inevitable, I do it slowly.
I've more than once been signaling a merge and had the car two lanes over attempt the opposite merge right into the space I'm signaling into, often when I'm already partially in it, without signaling. I'm very wary of merging next to someone now for that reason.