It's just how some Americans justify this particular measurement system.

As you said: it's what you're used to. I know 100F is a very warm day, but if someone said it was 50F I wouldn't intrinsically know how warm or cold that is. My brain works in Celsius, so someone saying its 20C I understand instantly.

Similarly length in US football fields - I don't know what that is, but it's second nature to someone who did grow up using them as a measurement.