Using a national currency as the de facto global reserve guarantees a trade deficit for that country.
No one else can manufacture USD's, so other countries have to acquire them by shaping their economies to supply goods and services demanded by the US. They can then use these earned dollars to transact with other countries, as the US itself insists they do.
For the US, this is a simple trade off - gain massive political influence (and market intelligence - all USD transactions go through US institutions regardless of where those transacting partners are located), at the expense of hollowing out domestic industry and running a deficit in physical goods traded.
The solution is a non-national global reserve, calculated on a basket of national currencies. This was Keynes argument at Bretton Woods, but the US would not have it then, and does not want it now.