We count the first day of the omer on the night of the second seder, yet some prominent, traditional haggadot do not include the bracha for counting (e.g., ArtScroll, Koren, Steinsaltz). Why not?
1 - Many people count in Shul right after Maariv. Although there are opinions that it is proper to say it at the end of the Seder (not to have two conflicting things - Seder / Omer) as far as I know the majority of people say it right after Maariv.
2 - Those living in Israel do not have a Seder where they say Omer at.