As far as I can tell, this is because Constitutional amendments are hard to enact (in the case of the EPA and Obamacare) and for reasons I don't understand, Congress doesn't seem to want to declare war any more.
Is that a fair assessment, or am I getting only part of the picture?
Federal laws (Obamacare/ACA) and federal entities (EPA) are created by Congress.
The executive branch (President), can issue executive orders, but they are limited to authority granted by Congress and the constitution.
The judicial branch is there to verify that the other two branches are staying within their constitutional authority.
Constitutional Amendments are difficult to enact, but that is intentional. Amendments limit government at all levels. Congress can't make any law, and the executive branch can't issue any order that conflicts with the constitution. The only thing you can do is amend the constitution again.
The problem is that Congress has become a gridlocked mess while also offloading a lot of it's authority to government agencies acting on it's behalf under the executive branch. Effectively increasing the scope and power of the presidency and by extension the reach of executive orders. This has also pushed the judicial branch into a much more active role than it was designed for.
Edit: It's also one of the contributing factors in the heated elections in the USA. As the power of the presidency grows, people become more invested in seeing their person in charge.
Agreed that declarations of war can just be declarations of war, no amendments required. It strikes me as odd that Congresscritters would be okay invading a country, but not okay with declaring war against them.
Obamacare (Patient Protection and Affordable Care Act) was also created by Congress - bill passed in both houses, signed by the President, upheld by the Supreme Court. That's basically a textbook example of how the system is supposed to work.
I'm not sure why the EPA (and others, like FEMA) were created by Executive Order instead.
As far as why Congress hasn't declared war since WW2, they've basically rolled up their say into the War Powers Act and the War Powers Resolution which provides for them being informed and issuing continuing approvals. They can then support the President (as Commander in Chief) but not officially declare war.