This Rashi (actually Medrash) begs the question why were the בהמות not blessed, would that include the snake (presumably no)?
See the Ramban who states that the blessing of fish and birds applies to בהמה and חיות. Since these are similar enough to bird and fish as those for example are moving souls in water/air and these are moving souls on the ground and are generally classified together as animals. Unlike trees.
However, the Sifsei Chachamim there explains that the animals are included in the blessing for man since they procreate like man - I suppose when compared to a tree.
Presumably then one could understand, that although perhaps everyone was actually blessed, just that the fowl and fish were blessed directly, and the land animals (ie: beasts etc...) simply were "included".
So now the question is only why did Hashem bless the fish/birds directly:
According to the Midrash: Perhaps only for us to learn a lesson from this incident. There might be other practical ramifications as well, like the 12 tribes, some are blessed directly and some are included. But I think the Midrash is highlighting one part.
Also note about blessing the animals:
The Gur Aryeh mentions that after the flood another blessing is given to the animals, this will cover the land animals; however, since the snake is now cursed it will remain cursed.
כׇּל־הַחַיָּ֨ה אֲשֶֽׁר־אִתְּךָ֜ מִכׇּל־בָּשָׂ֗ר בָּע֧וֹף וּבַבְּהֵמָ֛ה וּבְכׇל־הָרֶ֛מֶשׂ הָרֹמֵ֥שׂ עַל־הָאָ֖רֶץ (הוצא) [הַיְצֵ֣א] אִתָּ֑ךְ וְשָֽׁרְצ֣וּ בָאָ֔רֶץ וּפָר֥וּ וְרָב֖וּ עַל־הָאָֽרֶץ׃ Bring out with you every living thing of all flesh that is with you: birds, animals, and everything that creeps on earth; and let them swarm on the earth and be fertile and increase on earth.”
You can see that all animals are included.