Replace BTC with USD in your entire statement, and you'll have your answer as to why everyone (including governments) sees the USD as some sort of "magically universally accepted currency".
Totally. wrong. The magic words that exist on USD and don't exist on BTC are "legal tender". It means the bad boys with guns of the US government protect the purpose of your USD's on US soil, as a means of exchanging goods and services. In other words, currencies become accepted because regional powers are willing to kill, maim, and imprison if need be to make it so.
An additional point to this is: because the US Dollar is a currency agreed upon by the people of the US, through our government, as a medium of exchange for goods and services, it is in fact backed by the current and future productivity of the United States, which it turns out is still pretty damn valuable.