You’re presuming people are paying attention to everything all of the time, which they don’t. Humans need that extra level of redundancy to mitigate our own shortcomings. Also, we look twice right as well! Once while standing still, the second time instinctively when you’re already crossing in case something changed. Would you be required to do this 100% of the time? Probably not, but learning from a young age you can’t trust one look is good practice in more ways than one.

No. I'm assuming they don't

that's why I see no point to look left first