Even though the US Dollar is considered the “international” currency: that has limitations since people in other countries are not paid in that nor are they paying taxes in US Dollars (a British person is paid and taxed in GBP, a Japanese person earns money and pays taxes in Yen, so you get the point). You can go contactless but it’ll still convert it to your home country’s currency via forex in your bank account instead of paying that as it is.
You can be a US citizen earning money from remote work in Euros but one thing remains unchanged: your tax residency is tied to the USA meaning it’s subjected to taxes even if your earning money in another currency. You can tip servers overseas in USD but they still have to pay taxes imposed by their own country, and their wages remain in their currency (again, they do not earn USD in let’s say Korea, as they’re paid in Won).
Cryptocurrency isn’t recognized by the world bank, so that is out of the question. Instead, the main topic is about real money that’s valid for regular transactions. As in, why can’t you pay using $ in the UK? Is it because the £ is stronger? Why won’t they accept a $100 bill (£74) at a restaurant for a meal that costs £60 in Britain? If you opt for contactless, it’ll still convert GBP to USD anyway: so no currency is “region free” in that sense.
You just answered your own question. Currencies are regulated by their government and banks. If you want to have an international region-free currency, then no single government or bank should be able to regulate it, which is exactly what decentralised cryptocurrency is designed for.