I believe the designers are aiming to allow a wider range of characters to interact with each pillar.
While I see your point, I don't buy it
in this case anyway...
Not many PCs have had Sleight of Hand IME. I can't think of more than one, maybe two, cases in the last seven years of playing 5E because Thieves' tools proficiency, which every Rogue and at least one or two backgrounds IICR, was all you needed for picking locks and disarming traps.
Sleight of Hand is already useful in a variety of ways, giving it the "pick locks and disarm traps" makes it almost a must have skill, becoming worse than Stealth IMO.
Also, with 2024 rules, anyone can decide to have proficiency in Thieves' tools, so having the check be Dexterity (Thieves' tools) makes it simple enough if the goal is to allow a wider range of interaction.
They're not troubled by making individual checks easier, because they are considering the probabilities across the many checks that players will participate in.
No doubt. I expect the next version will remove skill checks altogether. PCs "failing" has already gone away in general, even with 2014, with "failed" checks becomeing "not making progess".
I think that by the rules it does apply.
Oh, certainly, by the rules they very much apply. Sorry if I was unclear, I meant I don't agree with that mechanic and don't think they should.
If you "need thieves' tools" in order to make a Dexterity (Sleight of Hand) check to open a lock, it makes sense to me you should
already know how to use those thieves' tools. Basically, the rule is granting advantage for what, IMO, should be the default condition, not the exceptional one.
Of course, the bigger question is: if you're not a Thief using Fast Hands, DO you roll sleight of hand? Or just dexterity? Nobody knows! The books don't say, therefore we're left with the Tools section, that we KNOW doesn't give complete instructions.
Well, according to all the other sections addressing locks, for instance, if you are not a Rogue (Thief), you:
The check is Dexterity (Sleight of Hands), when using thieves' tools, to open a lock.
Do you need Sleight of Hand proficiency? Of course not. If you have it, you add your proficiency bonus as usual. That's all.
Do you need proficiency in thieves' tools? Of course not. If you have it, you add your proficiency bonus as usual. That's all.
The rules are clear on this. If you have
both, instead of adding your proficiency bonus for each, you add it for one and gain advantage.
Where it becomes unclear IMO is "do you (actually) need thieves' tools"? Can you just make a Dexterity check without thieves' tools? What about with improvised thieves' tools?
I would rule you cannot make the check without Thieves' tools, or at the very least make the check with disadvantage if you are using improvised tools (like the slim dagger).
It's a mess, and for the DMG's traps it's a moot question anyway: thieves' tools don't come into play at all. Does it matter if you would roll dexterity or sleight of hand, with advantage or without, when you don't get to roll at all?
Yes, when I finally read that I found it patently absurd!
When it comes to disarming a trap I don't recall you having to roll at all. You roll to see the trap and/or understand how it works, then it is just a matter of cutting a wire, using an iron spike, etc. all without the need for a check! Pathetic and lazy game design IMO! Just another way to move the game away from the exploration pillar...
I wanna make it very very clear that, whether we agree or not, this isn't on us. We're trying to make head or tales of incomplete rules. This is on them.
Welcome to 5E!!!

It has always been:
Rulings over rules, make up whatever you want, why should we be bothered to come up wit concrete mechanics you can actually
use and change if you don't like. It is much easier for us to just handwave things and claim "natural language" and fill pages with artwork than create a solid game system with fleshed out mechanics.
As for whether the advantage (and generally mixing skills with tools) is a good rule, I'm still ambivalent, but you're making very good points. I'll have to think about DCs before making up my mind, another thing 5e didn't grace us with. Because at the end of the day, whatever the formula of rolling is, the goal is to produce a probability of success that makes sense, depending on the character's abilities and the situation.
For myself, I find the 18 skills too universally applied. I prefer tools to take the roles for which they were intended.
If someone has proficiency in the herbalism kit, they know about herbs and such. Do they know any or all of the other things associated with Nature or Survival skills? No, the know about herbalism stuff.
Does someone with Nature or Survival skills know enough about herbs and such to make a
potion of healing or similar things? No. That is what the herbalism kit proficiency is for IMO.
Who cares? Houseruling is free. There's exactly one group of people that has to accept the changes we make, and it's the group we're playing with.
Oh, I can houserule things of course and I will. But I care because poor game design leads to issues like those raised in this thread. For all people love 2024 rules, they are colossal fails in there which for me are unforgiveable at this point in D&D design.