Despite the 26-year-old's inspirational performances and an injury crisis, Lippi looked elsewhere when picking his squad for Saturday's trip to Montenegro and the home match with Ireland on April 1.
"You know I only talk about players that are here," Lippi told a news conference after the Group Eight leaders held their first training session at their Florence base.
"I don't have to give reasons for my decisions. I have my convictions and I don't have to give explanations. And that's not because I'm arrogant."
The former Bari, AS Roma and Real Madrid forward had a reputation for causing trouble on and off the pitch but has matured hugely in his two seasons at Samp, leading Italian media outlets to launch a campaign to get him in the national side.
He was in Roberto Donadoni's Euro 2008 squad but Lippi has continued to overlook him alongside 34-year-old Juventus striker Alessandro Del Piero.
Cassano's Samp team mate Giampaolo Pazzini, included in an Azzurri squad for the first time, defended Lippi.
"It is easier to score than in the past thanks to Cassano's passes but I've also created a lot of goals myself," said Pazzini, who has scored eight in eight league games after a January move from Fiorentina.
"Let's remember that Lippi is a world champion and it is up to him to make the final decision. For me to reach the national team is a great satisfaction. I will try to keep hold of this shirt. I don't hold any grudges against Fiorentina."
Pazzini was frozen out at Fiorentina following the close season arrival of Alberto Gilardino from AC Milan. The 24-year-old has made it into world champions' squad largely because of an injury to Gilardino.
