“But what can possibly be so different between the states, other than peoples’ opinion about law enforcement?”
Some states have different proportions of people whose families originate in different cultures, and history, conditions, and whatever can lead to different opinions and traditions.
“Isn’t all it takes, one decision from above, and the states will have to follow whatever the new law the nation settles down with?”
Yes, that’s exactly the problem with Federal laws. What people decide ought to be legal or illegal is not easily agreed upon even at a local level, but on a national scale it’s even harder and also much more unfair and unavoidable for dissenters if you try to force one law for an entire continent. Examples: Abortion, marijuana, alcohol, fireworks, weapons, vehicles, legal means of self-defense, gay rights, sexual rights, school issues, other moral questions.
“And doesn’t it bring people in different states into unevenly beneficial positions in terms of law?”
Yes, in some cases. But we have at least a historical principal that laws are “by the people, for the people”, so it also provides different perspectives and room to experiment and compare different laws. For example, in Washington state we have a high sales tax (like VAT) but NO state income tax. In adjacent Oregon State, they have NO sales tax but an annoying (in the US people fill out their own tax forms) personal income tax. (And yes, Washingtonians go buy stuff in Oregon to avoid the income tax.)
“It seems like there’s a firmly rooted consensus among the americans I’ve been talking to, on this topic. Is there really no opposition for different laws?”
There’s a ton of opposition, from people in power in the Federal Government. And from some interest groups that want to impose their ideas of the “right” laws on the entire continent.