Needless to say, I disagree with pretty much all of this. I think all of those things do exist, at least in adequate guidance. If not, then wouldn't you think it would be a pervasive problem among gamers? Your complaints are almost non existent among the millions of gamers who play. That tells me that those problems aren't really problems as a whole. I get how it doesn't meet what you want, and that's OK, but that doesn't mean the game is lacking in those areas like you claim. I.e., your experiences are hardly universal or evidence that the game in general is lacking.The example I gave in this thread. Skill use. There's very little guidance on how to adjudicate the skills, on what they actually do. Or even using ability scores in general. Or how to bring magic items into a game. The prices listed are several thousand gp wide, and the advice given on how many to give players is "use your judgement"--but there's no advice given on how to develop that judgement. A post earlier suggested that the issue is really with the DMG, where the guidance should be, but is absent. It's fine to say "use your judgement", but without helping the DM develop that judgement, it's useless advice.
The game does tell you what ability scores are, how they are defined, and how they are used (by giving modifiers, how they are generated, and telling you what types of skills would be related to each one). I'm not sure what more you could need. Same with skills. Same with magic items (it tells you how items are sorted by rarity, and what levels PCs typically tend to be when they find items by said rarity). The DMG literally includes guidelines on everything you just listed, so I have to ask, did you even read it?
If you're looking for detailed black and white hard lines for every possible scenario, you're not going to get it. That would result in a book 1000 pages long and even then things would be missing. D&D learned the hard way that when you put things like "player of level X will have this exact amount of magic items of this type' and "skills are used exactly like this and only like this", you end up with players who rely on that and can't or won't think creatively. It's the entire philosophy of "anything not expressly prohibited is possible." As I mentioned, what you described as wanting feels like shackles. A prison. "If there isn't a specific rule for it, you can't do it."
And I for one am glad that D&D got away from that. Judging by the sales #s, so are most people. But either way, none of that means that the rules are missing as you claim. I just reopened my DMG and yup, everything you claim isn't there is. Guidelines and rules for how to handle skills, ability scores, magic item frequency, etc. The 5e DMG is actually quite extensive. In fact, I ignore about half of it because I already know how I want to handle it in game.