xkcd #3232: Countdown Standard
Title text:
Anyone who is caught counting 'three ... two ... one ... zero ... GO!' will be punished with a lifetime of eating only ISO standard food samples.
Transcript:
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Source: https://xkcd.com/3232/
I think it's so the person being lifted doesn't tense, when ready for it. So you kind of surprised them but it's an acceptable surprise, as they are expecting a move.
These are usually either unconscious or screaming people, not the kind to care about what an EMT says.