

Essentially Congress got tired of having to do actual work because it was taking time away from them soliciting bribes from corporations, which is the profitable part of the job, so they abdicated most of their responsibilities to the president.
Constitutionally the President doesn’t really get to do much except work out the administrative details of everything Congress has ordered him to do. Since Congress abandoned that responsibility the president has a lot more control.
Trump still doesn’t actually have the authority to do most of what he’s doing, but he’s abusing emergency war time regulations as justification despite this not actually being an emergency or war time. Everybody is just too chicken shit to call him on his bullshit though because then they’d have to own up to all the times previous presidents over the last few decades abused the same loopholes.
Republicans have spent decades building up a party that values loyalty to the party line above all else and that regularly uses propaganda and outright lies to the public to manufacture support. Democrats have spent decades carefully crafting their political stance to be as mild and non-threatening to the rich and powerful as they possibly can.
Republicans won’t oppose him because disloyalty is political poison to them, and Democrats only provide token resistance because Trump still has rich donors backing him.
All the US checks and balances are predicated on the idea that no one party could take control of all three branches of government, and even if they did that the people in those positions would hold their duty to the law above party loyalty. Trump has successfully proven both ideas invalid, and his only opposition is entirely too milquetoast to do a god damn thing about it.