If it's a period in which we don't have any sound records (a.k.a. most of history), it's impossible to tell what their voices and dialects sound like. It's challenging enough to guess what they talked like with any accuracy in a film, much less how the voices sounded with zero audible evidence.

Also, having well-known actors doing foreign accents can be risky, since audiences know what the voices sound like, and the effect of a star using a dialect can be utterly laughable. Example: Harrison Ford as the Russian sub commander. Another example: Tom Cruise in Valkyrie was going to try the whole film with a German accent, but the filmmakers wisely said, "Fuck it."

So what do you do? Instead of having all these British and American actors take a stab at doing an ancient Roman accent and risking folks laughing at well-known actors doing silly accents, you let them use their own voice. It's a lot easier, and since often you're not using the same words and languages that they would've used in that period, why go for historical accuracy on the accent?