Because humans are not good at eye-balling distance, especially while moving. The only other point of reference is going to be the only other thing travelling on the roads at a similar speed... cars. Hence, the rough comparison of "car-lengths".

I use the 3 second rule...

It isn't really car-length, is it speed-adjusted to scale larger at high speeds and closer at low speeds.

But the display on the screen shows a car with a line and the numbers 1-7 as you adjust it so people shorthand refer to it as "car-lengths"