Why isn't 5. f4 considered a strong move in the main line French Advance, after
[fen ""]
1. e4 e6 2. d4 d5 3. e5 c5 4. c3 Nc6
Everyone plays 5. Nf3 here, but isn't it logical to support e5 with a Pawn and later 6. Nf3, as it is for example in the Steinitz Variation of the Classical French, after
[fen ""]
1. e4 e6 2. d4 d5 3. Nc3 Nf6 4. e5 Nfd7 5. f4 c5 6. Nf3