Not just remain hidden but try to hide as well is what you fail to understand or accept. Additional condition is if as you say these features can only be used when obscured in their respective fashion while not seen byn prying eyes starring at them, then its additional conditions since anyone can try to hide when not seen anyway!!
Let's read again: ''Naturally Stealthy and Mask of the Wild—do allow members of those subraces to try to hide in their special circumstances even when observers are nearby. Normally, you can’t hide from someone if you’re in full view. A lightfoot halfling, though, can try to vanish behind a creature that is at least one size larger, and a wood elf can try to hide simply by being in heavy rain, mist, falling snow, foliage, or similar natural phenomena.”