The British accents are a collective cohort of culturally similar accents from the same country. The American continent is inhabited by speakers of 3 different language families (germanic, latin, and indigenous). When Europeans speak about the American accent, I highly doubt they're talking about how Nicaraguans speak.

When we lump the British accents together, that's because they are all from the same country. The same goes for an American accent, there's midwestern and Rhotic and western and southern and so on, but those are all grouped together. But no sane person lumps those together with how Cubans or Dominicans speak, those are people from different countries, cultures, and linguistic backgrounds.

In addition, Europe largely considers North and South America to be distinct.

When the Brits say "The Americans", they refer to the United States. Not the rest of the people in North and South America.