DFA and RBI have become their own words, where you read them as letters, but together the letters make a single “word”.
If you think RBI should never have an “s” after it, then you are always reading it as “runs batted in” instead of “ARE-BE-EYE”. And if that’s the case, then when it’s singular, you would say “he got a RBI” instead of “he got an RBI”, and that would be ridiculous.