I can't quite tell from your tone if you're implying that this is a bad thing, or if you're of the opinion that Congress is shirking on their duties when they do this. I read it like, "'Congress just LOVES to pass vague laws so others have to step in' /sarcasm." My apologies if I'm misreading your tone.
For posterity, just to be clear, this is definitely a good thing here. In the case of, for example, the environment, which this post is about: Members of Congress are not experts in every field, so it makes more sense to pass a law called the Clean Air Act requiring protection of the air with some parameters, and then letting the experts at EPA fill in the details. In this case at least, it's not Congress shirking their duties, but rather the way it should work. i.e. Congress issues laws requiring general things, and the experts at the various federal agencies fill in the details with regulations (though obviously it doesn't always work perfectly).
Also, I don't think administrative agencies are constitutional. They're a fourth branch of government not described in the constitution.
Regarding your second point, at this point that's not really debatable. SCOTUS has consistently ruled that admin agencies are constitutional within the legislative intent of Congress. Where agencies act ultra vires, beyond their power, SCOTUS will frequently rule what they did unconstitutional. But they are in themselves constitutional, and saying otherwise smacks of the same arguments people make when they say that income taxes are illegal. i.e. it discredits the person making the statement.
There is a difference between what the courts have ruled and what the constitution actually says. The significance of the difference being that when the courts have it wrong, you can sometimes convince them to change their minds. Or at some point different justices are appointed with different opinions. Roe v. Wade is long-established precedent but that doesn't mean it hasn't been a major consideration when appointing justices to the Supreme Court.
And there is a completely reasonable way to have "administrative agencies" without making them part of the executive -- you make them part of the legislature, in the nature of the CBO. The difference being that after they propose new rules, Congress would vote to approve them or not. Which in most cases could be a rubber stamp but it would give Congress the opportunity to say no when it matters, and get the executive branch entirely out of the rulemaking business.
Obviously the practical needs of Congress has no bearing on whether a delegation of legislative power is constitutional.
In the current political environment that's not really an option, so you pass on power to the executive and states to make changes instead.