There’s a huge part of it that is because there’s no communication mechanism to resolve conflict on the road, really.
If you’re stuck behind someone going slower than you want to, how can you clearly communicate that?
Even in a world where you could clearly speak to them, there’s huge potential for disagreement there, and since the only communication mechanisms can very easily be construed as needlessly dangerous or aggressive, of course you’re gonna see frustration and anger as a result