Why is that absurd? I'm with the other people who replied. If someone is standing in front of you making no attempt to hide, they can be seen. Otherwise the world would be a weird place with people randomly vanishing. Eyesight could not be trusted.
This doesn't need to be a rule. The authors of the rulebook felt there were certain things common sense enough not to have to publish in the book. One of them is that people can see normally if nothing is changing the situation.
What if that someone is invisible and sitting on a stone bench in the corner quietly reading a letter laying on the table in front of them? They are not making any attempt to hide, they are sitting right in the open and just happen to be naturally invisible. But someone else in the room would have to extremely attentive to notice the faint sounds that indicate that someone is sitting there.
It is completely absurd to assume that anyone and anything not taking the hide action is automatically detected by everyone else. Most of the time they will be but not always. How do you set the DC to detect them? Use the guidelines in the rules to figure out if it's a easy, normal or hard task depending on the environment and the kind of clues that exist.
And BTW, eyesight really cannot be trusted. Your mind plays a lot of tricks with what you think you see, and your memory will further mess with what you think you saw even a short time ago.
