An opposite question is interesting: why not abolish state governments and only have the federal government? I haven’t thought about it deeply so don’t have a strong sense that it would be better than the status quo, though I suspect it might.
Centralization and size comes with inefficiency and slow decision making. It also comes with economies of scale. For something like the Medicare, the economy of scale dominates and it makes much more sense to be federal. When it comes to running schools, there really isn't much economy of scale so it makes much more sense to run it locally.
Economies (and diseconomies) of scale exist, of course, but aren't central to what is the responsibility of the federal government and what is the responsibility of the state governments. That, of course, is laid out rather plainly in the founding legal document of the country.
The more people are ruled by a single government that is ostensibly a representative democracy, the less representative said democracy actually is. This should be rather evident - you can't grow parliament size indefinitely if you want it to continue functioning as a deliberative chamber, which in turn means that every parliamentarian represents more and more people, which in practice means that they don't represent them well.
The Civil War settled that the feds could rule over the explicitly authorized powers of the federal government. In the 30s the feds reneged and decided blatantly unconstitutional stuff like the (later passed) Civil rights act could hold if they just call everything interstate commerce.
When the US falls apart this will be a likely central focus .
CRA under the 1875 version was found unconstitutional per the Civil Rights Cases of the Supreme court [0].
In the 1930s the executive threatened to pack the supreme court and many analogous private/intrastate commerce regulations were allowed federally by this bastardization of 'interstate commerce' by the court. Then they repassed the CRA with yet again stuff found unconstitutional, using their new version of 'interstate commerce' (everything).
The CRA objectively has been found unconstitutional by the Supreme court, and I believe it may be again under the latest generation of the court.
so, probably restating what you said, if someone wanted to pass something like that, a constitutional amendment is the better way since relying on interstate commerce clause is shaky ground... is that right interpretation?
Indeed. It should be noted that the Commerce Clause underpins most federal powers at this point. This is one of the big reasons why it's such a sacred cow - its ridiculousness is rather obvious, but all players have important things that depend on it that they don't want to risk losing.
