Interstingly it is the same in german, you would say "Er ist Arzt" not "Er ist ein Arzt". Which makes me wonder why english uses an indefinite article there, since it doesn't seem to be either germanic or romantic in origin.
Eh, Italian has it tho.
"(Lui) è un dottore", "(Io) sono un dottore".
"Un" is the indefinite article.
Subject is usually dropped, thus the brackets
This was the root of the overblown Kennedy gaffe where he said “ich bin ein Berliner” instead of “ich bin berliner.”