Why are the courts & Congress so slow to rein in the President?
The judicial-review process is slow by design. Executive actions are biased towards acting quickly and with clear direction. This is important in a crisis, or when "someone needs to make a decision".
The legislative and judicial branches are biased toward deliberation and consensus: In court cases, both sides get ample time to present evidence in their favor, to rebut the evidence of the opposition, to argue about definitions and standards and applicability. In legislative debates, there is additional casting about for alternative approaches, additional research, etc. Judicial processes are also reactive - they can't anticipate problems or devise / enforce policies beyond the matters that are brought to them.
This means that an openly renegade / hostile / criminal Chief Executive can do a lot of damage very quickly, and even if the ultimate resolution / authority / legitimacy eventually rules against the Executive, there will be an extended period where Legislative & Judicial organs are struggling to keep up, to prioritize, to come to consensus, to make sure they have all the evidence.
Unitarity Executive theory, beloved of the conservatives in SCotUS & MAGA, is that the President should therefore be able to do whatever he wants. But this ignores the solid strengths of other branches (i.e. the improved confidence & legitimacy that comes through research & consensus). The current President has done very well on the shadow docket, where his Unitary-Executive fans on SCotUS have given him plenty of freedom to set policy as he wishes. (Their hypocrisy is noted though - they did not provide the same freedoms to his predecessor.) He has done less well on formal, argue-and-discuss cases. He has done terribly in lower-court rulings. He is doing far less well in the court of public opinion, which is the ultimate source of legitimacy for any government official. As the damage done by this Administration builds up - due to inflation, poorly-chosen conflicts, its internal contradictions, its embrace of corruption, and the finite lifetime of the President, its unpopularity & internal divisions will grow.
At some point, "Unitary Executive" partisans need to confront the questions "What if we got the wrong guy?" and "What if he decides he doesn't need us any more?"