she wasn't though, that's her first time on the outside, so she's not used to fighting everyday for her life. Aside from that, you would have to be very stupid to whistle, and sing loud in a world filled with clickers, which of course rely on sound. when joel told her to watch her step crossing the building, she gives him a look like please this is nothing, i understand what naughty dog is going for, i'm a cool bad ass teen, just would like a more serious tone in my survival horror games, i feel like joel captures the feeling of the miserable world they are living perfecty.
Man oh man, you are missing so much subtext. The moments of brevity aren't just there for the sake of it, they're there to show there's still room to breath in this world (hope). When she walked across the plank, it wasn't so show she was a 'bad ass' teen for goodness sake, it was to show she was single minded and independent (Sarah buying Joel a watch and insisting it was 'still today'). His daughter was like that and it reminded him of his daughter. That's why he looks at the watch.
The PS5 Exists.