English has the concept of a "determiner", or a word in front of X to separate X from a bigger group. Articles are determiners.
When you are talking about X in general, or all X, or any X, the determiner is not needed unless you want to emphasize that fact.
Why do you think Italians are so good at football?
We don't care which Italians, so no determiner needed.
Why do you think the Italians are so good at football.
Here, we are separating "Italians" from a bigger group. The bigger group here is not in the sentence so without additional context we have to guess. Likely the bigger group here is multiple or all possible ethnicities.
Americans speak English differently to the British.
The speaker here feels no need to talk about a separate group of Americans, he/she is talking about Americans in general. The speaker doesn't care which Americans.
With regard to British - there's a couple things going on:
Does the speaker care which "British"? Probably not, but the is emphasizing that "British" is a separate group from Americans.
English really prefers singular nouns to have determiners or articles of some sort, because when we talk about a singular noun 99% of time we have a specific instance in mind. To talk about a noun in general the plural is usually used.
American can be both a noun and am adjective. British is only an adjective. Adjectives can be used as nouns, e.g. the red = the red one or the rich = the rich people. But you really need the article to signify that it's a noun in that case.