Europeans have been settling in North America for 500 years. The United States being a young country has nothing to do with the evolution of accents and dialects. When the US was formed the Spanish had been in the Americas for 200 years, the French and English not much less, in addition to enslaved Africans who brought their own native languages to the continent and then were forced to learn English, Spanish, French, or Portuguese. That alone is more than enough time and groups of people for dialects and accents to develop.
Europeans have been settling in North America for 500 years. The United States being a young country has nothing to do with the evolution of accents and dialects. When the US was formed the Spanish had been in the Americas for 200 years, the French and English not much less, in addition to enslaved Africans who brought their own native languages to the continent and then were forced to learn English, Spanish, French, or Portuguese. That alone is more than enough time and groups of people for dialects and accents to develop.