How did R2 and C3PO not "sense" the sand people right in front of them??

The Stuff was used in a horror film from the 80’s. It’s a decent flick.

And yes, the droids should have known about the Sand People. I assume they have ways to sneak around so that droids won’t detect them? Or perhaps since C3PO is programmed for etiquette and protocol, he has no sensors to detect lifeforms as it isn’t pragmatic. R2 units are programmed for being co-pilots and computer hacking. I guess they aren’t as good at picking up nearby lifeforms. He couldn’t sense Yoda either.

R2 basically had RADAR, and the arrogant Sand People would've thought nothing of marching right at their target without thinking of hiding, unlike the Jawas earlier on in the movie who showed up intermittently enough on R2's radar to worry him, but were invisible otherwise.

R2 was not with Luke and C3PO, he was down by the speeder. And ,it seems, C3PO had no such scanners and only R2 did. Also there is nothing to indicate that R2 could have given proximate distances; just that there was creatures approaching them.

The fact that you refer to the force as "the force" is an indication of its staying power. Something too fantastical or mystical sounding might not have been nearly as catchy or memorable. If it was called "Quee-gin-klow" it would not have been nearly as iconic or referred to as often. calling the mystic energy field of the star wars universe "the force" is perfect in its simplicity. And it is still mystical sounding enough to be believable; you are looking at it through post star wars lenses that is why it sounds less mystical. Calling it "the force" in the 1970 was pretty unique and mystical by those standards. Star Wars changed conventional movie/fiction tools forever.