Short answer: because car drivers are arseholes.
Long answer: I’m cycling home, and there’s a pinch point where the kerb on each side renders the road wide enough for one car only. And, since I’m cycling—and because I’m cycling defensively, i.e. taking the lane rather than cowering within a centimetre of the kerb—the car behind can’t get past until the road widens again in about 20m. The **** driving the car beeps.
What am I supposed to do? Not exist? (Hint: the correct answer is, apparently, yes.) Should I stop, mount the kerb, and defer to my master in his automobile? Does owning a car also give you ownership of the road?
As the road widens, the car pulls past me. The driver slows and enquires threateningly:
Do you want to get killed?
Fuck this Nietzschean bullshit.
That’s why people don’t cycle more.