State laws. Like when something legal in one state is illegal in a neighbouring state. How can you reasonably function unless you never move within your country. Also state taxes. Also tips.
It takes about 8 to 12 hours to drive across Texas, which is longer than it takes to cross Germany. France and Germany have different laws, and so do Texas and New Mexico. Once you realise that the USA is essentially 50 different countries stapled together, it makes a lot more sense.
Texas has a bigger population than Australia. In fact, it would be 51st largest population and 40th largest area in the world if it split off from the USA.
Not so sure about this.....maybe you could find two states that have very differing laws compared to France-Germany, but on the whole it's probably not true. Kind of hard to quantify that though.
It works the same way it works in Europe between countries in the EU. The laws are similar enough that it isn't a problem in most cases. There are also efforts that states do to harmonize parts of the law. It keeps the peace because states have different ideas on how to govern themselves.
That then plays out to taxes, as some states have a far more active government than others. So, the states, counties, and municipalities have their own ways to generate revenue to provide the services their communities want.
That is actually pretty rare in the USA. If anything, Americans are very mobile within the country. It is common for people to live in multiple states within their lifetime.