Because "See Invisible" exists. As mentioned by Treantmonk, technically being invisible vs a creature with See Invisible still allowed you to have the benefits of being invisible (advantage to hit, disadvantage to being hit), despite being seen. This closes that loophole.
Personal view on some of this:
Say we're in a forest, and a rogue slips behind a tree and tries to hide. He has total cover at the moment, so he can attempt the Stealth check. He makes the roll, making him invisible. What impact does that have?
Well, if he loses the status as soon as he's in line of sight again, and there's nothing providing additional obscurement other than the trees, he's basically stuck staying behind that one tree.
However, by making him "invisible", we can now use the trope of him moving from tree to tree, carefully avoiding any enemy's sight, until he can sneak up behind the shaman that is directing the battle, or whatever.
This, to me, feels like the old Eagle Totem flight shtick: You can fly, but have to land at the end of your turn. In this case, you remain invisible, as long as you end up out of direct sight at the end of your turn. You can move in the open by taking advantage of lapses in attention — the chaos of battle, the guards talking to each other at the gate, the flow of the dance at the ball, etc — and be "invisible" because you aren't being "seen".
You lose the invisible status when you attack or do something else to draw attention to yourself.
In another situation, if a guard is guarding a warehouse door, standing like 10' in front of it, and you walk straight up to him, you can't possibly remain invisible. But if you carefully edged along the wall behind him, you might be able to avoid his attention and slip through the warehouse door, even though you're technically within range of his sight, given the usual, "Anything within 360° is fair game"-approach to visibility.
In a way, it's to allow for some of the tactics that facing rules might allow for, without actually having to implement facing rules.
And while it might not be part of the rules, I personally would say that a passive perception that beats the stealth roll of the rogue does not automatically reveal him; rather, I would instead say that a passive perception beating the stealth roll indicates to the creatures that something is amiss, and thus given them a reason to use the Search action.
In addition, there's a difference between giving up stealth entirely and just walking up, and trying to move stealthily towards a creature that happens to be looking your direction. If you're trying to maintain stealth, even taking advantage of lapses in attention to move towards someone who's looking your way, I'd still expect you to end your movement in a position where you can't readily be seen (ie: at least partial cover, dim lighting, light obscurement, etc). Being in direct view, completely out of cover and in bright light, when the creature's turn starts, I can't imagine not leading to the rogue being spotted.
A stupidly high stealth roll may let the rogue slip from potted plant to trophy pillar to tapestry as he sneaks up to the guards watching the king's bedroom door, but thematically, I'm OK with that. Sometimes players get to do crazy, over-the-top stuff. And I think treating being hidden as "invisibility" allows for that sort of thematic play.
Overall, it seems to be written to let you do cool stuff, as long as the GM keeps a handle on people trying to do stupid stuff.
