The text talks about taking of life, liberty, and property. That's a lot of ground to cover. Implicitly, the government has the power to take your life, etc., but some process is due before this can happen.
Nothing in the constitution is self-executing, so in that sense 5A doesn't act as a factual impediment; it's remedial. So there are tons of areas where the government shouldn't tread, and we all know this (9A, 10A), why does SCOTUS say this is a 5A substantive due process issue?
Ultimately, because they're infringements on liberty. And, to the framers, liberty was an essential part of being human. Now, the government can infringe on parts of liberty, if due process is followed. Is there a core, however, that is just beyond government intervention? See, e.g., anti-suicide laws: the government infringes there.
Note that 5A more or less permits the death penalty (the text acknowledges life can be taken). So if we want to find limits on the government's powers, it's got to be in liberty, not life.
No easy answer here. We just have some fundamental rights, that SCOTUS has held either that can't be regulated, or there must be a balance of individual liberty interests vs. social interests. So, for example, under some circumstances the government can limit the distribution of contraceptives, but not in all (see Griswold v. CT).
If the government can always find an interest that always trumps a balancing test, is there really a well-protected system of rights? The courts could maybe decline to establish a balancing test in some cases (Marbury asserts they have this power).
So if there are fundamental rights that the courts find, and the have the power to overturn their own precedent, they could un-fundamentalize them. So doesn't that mean they're not fundamental?
But of the available options, the courts are appealing. If we turned this over to the legislature, we've got the November 2 problem. If we give it to the executive, then rights change with administrations. So with SCOTUS, we at least get some institutional stability.
Scalia's formulation: there are certain things that the constitution puts beyond the reach of the political process. There's a tacit assumption there-- that everything else is available for handling by the federal government. The premise, in other words, is that federal power is plenary, unless the constitution has taken something off the table.
The Bill of Rights takes things off the government's table (or at least the federal government's table), and makes sure that they stay on the individual's table. So, even though core rights may be dwindling, or shifting, they at least exist.
Anyway, this is why prohibitions on coerced confessions are lodged in substantive due process, not the self-incrimination clause: freedom of conscience, overriding the will.
This is how we come to coerced confession (with Brown v. Mississippi, in 1936). There had been a long tradition of knowing about "the third degree" (a term taken from the rigorous rite of examination to become a 3rd degree Mason, which came to mean the use of physical pressure to produce results in interrogation). In the early part of the 20th century, the third degree was seen as a sign that the police were doing a good job: it was reassuring that they were doing this. It was widely accepted and the term "the third degree" didn't have a negative tone to it.
In conjunction with the rise of organized crime, President Herbert Hoover tasks the Wickersham commission with looking not only at organized crime, but also at police lawlessness. And the commission finds rampant lawlessness (not seeking warrants, holding people incommunicado, physical violence); that's in 1932-33.
The facts in Brown are absolutely undisputed. They were largely testified to, even. So the question is really just "is this OK?" And it's just not: the torture is pretty extreme, plus threats of death just for good measure. John C. Stennis was the prosecutor, just for interesting trivia: he goes on to serve 42 years in the US Senate.
So, because the facts are so extreme, the court is able to find that there was a violation of due process (14A, so it applies to states). The emotional thrust of the decision is basically, "you can't do that.
Quickly, coerced confession cases start using language like "rational intellect," "overcoming free will," etc. But the bottom line is that as a matter of substantive due process, you may not coerce confessions.