The rules for hiding and stealth in 5E are not at all ambiguous.
The rules on stealth are crystal clear
From the Basic PDF, p 60: "You can’t hide from a creature that can see you".
Does that mean that you can only hide if you are invisible, or the people you are hiding from are blind? That's the most literal reading.
The rules go on to reinforce this literal reading by mentioning invisibility as one important way to facilitate hiding. The rules also go on to reference the concealment rules in chapter 8. And p 65 tells us that things that are lightly obscured can be seen, but with disadvantage on the Perception check. Which seems to imply that a stealthy rogue can't sneak up on someone through patchy fog or moderate foliage. This implication is reinforced by the rules calling out, as a special ability for wood elves, that they can hide when only lightly obscured by fog or foliage - though this itself is ambiguous (does it mean that elves are invisible even in light mist or moderate foliage? or is it an exception to the "can't hide from someone who can see you" rule?).
None of this seems to involve "rulings not rules". It looks like an attempt to write strict rules which are very punitive for non-elven rogues.
There are sources of confusion, however. For instance, the rules for concealment on p 65 don't mention cover or obstacles at all. They mention lighting, foliage and mist/fog. Which leaves it uncertain how the hiding rules are meant to interact with physical obstacles like walls and furniture. This uncertainy is compounded by the rules for
total cover on p 74, which say that "A target has total cover if it is completely concealed by an obstacle." How does the use of the word "concealed" on that occasion relate to the rules for concealment on p 65, which could have mentioned, but don't mention, the possibility of physical obstacles providing concealment?
Another source of confusion is the reference in the hiding rules on p 60 to distracted creatures. In terms of the rule that you can't hide from someone who can see you, this barely makes sense, because a distracted but sighted person has the capacity to see a person sneaking up on him or her, but won't and doesn't because s/he is distracted.
Further confusion and ambiguity is introduced if we move beyond the actual rules themselves to procedures of play. For instance, if a distracted creature is someone who can be hidden from, then if my PC is very quiet presumably s/he can sneak past someone who has his/her back to my PC. So who gets to decide whether or not a given NPC has his/her back to my PC? One way to handle this: the GM gets to decide, therefore (in effect) opening up or shutting down the possibility of an attempt to sneak past. Another way to handle this: the player gets to roll the check, and if it fails then one option for the GM to explain that failure is that the NPC turned around and saw my PC trying to sneak past.
It's not just that the hiding rules don't tell us which procedure is to be preferred - they don't even canvass that a given table has to make a decision about such things.
For a point of contrast, look at the hermit rules for the "discovery" background feature, which tell the player "Work with your GM to determine the details of you discovery and its impact on the campaign". Those rules actually talk about the need for the people at the table to make a decision, even if they don't specify the mechanism in much detail. The hiding rules don't. They are poorly written.
WRT the discussion about stealth, I think some people think that excessive clarification ends up as an attempt to take the DM out of the picture. As someone mentioned earlier, there are potentially an almost infinite variety of situations in which someone could try to hide, and it's pointless to try to cover every possibility with explicit rules.
I don't dissent from that. But given that what you say is true, I also woudln't write stealth rules that begin with a blanket statement that you can't hide from someone who can see you. That statement is in direct contradiction to the notion of "infinite variety".