Abraham and Tomori both featured in Chelsea's 2-1 Champions League victory at Lille on Wednesday, with the former scoring the opening goal on his birthday.
Both players are also eligible to play for Nigeria and Abraham said he remained undecided about his international future before the squad announcement.
"He is the leading English goal scorer in the league," Southgate told a news conference. "He is a player that we put into the squad very young at a time when we wanted to have a look at some younger players and change the complexion of what we were doing.
"It was a bit early for him and now it is brilliant the opportunity he has been given by Frank [Lampard] along with some of the other young English players at Chelsea, it has been fabulous for them.
"We have got hundreds of players in our system now that are dual-nationality so I don't think it's ethically right to give a player a cap just so they cannot play for another country.
"The two boys are in the squad on merit. I'm not doing to guarantee that they will get on the pitch."
However, Manchester United attacker Jesse Lingard has been left out as well as Tottenham's Dele Alli, Liverpool's Alex Oxlade-Chamberlain, while Manchester City defender Kyle Walker has also been omitted.
Walker was also left out of the England squad in August and Southgate said the form of other players in why he overlooked the City defender.
"It is getting increasingly difficult to pick a squad because the competition for places is more and more intense," Southgate added. "There have been some difficult decisions to make.
I was really happy with what Kieran [Trippier] and Trent [Alexander-Arnold] did in the matches. They're both in good form for their clubs. Kyle [Walker] is also in good form for his club.
"I think he's playing well. It's not the case that he's playing poorly, we've just got really good competition in certain positions on the field."
Leicester midfielder James Maddison has also been included after an impressive start to the campaign, while Aston Villa defender Tyrone Mings retains his place, having been called up for the first time in the last international break.
Full squad: Tom Heaton (Aston Villa), Jordan Pickford (Everton), Nick Pope (Burnley); Trent Alexander-Arnold (Liverpool), Ben Chilwell (Leicester), Joe Gomez (Liverpool), Michael Keane (Everton), Harry Maguire (Manchester United), Tyrone Mings (Aston Villa), Danny Rose (Tottenham), Fikayo Tomori (Chelsea), Kieran Trippier (Atletico Madrid); Ross Barkley (Everton), Fabian Delph (Everton), Jordan Henderson (Liverpool), James Maddison (Leicester), Mason Mount (Chelsea), Declan Rice (West Ham), Harry Winks (Tottenham); Tammy Abraham (Chelsea), Harry Kane (Tottenham), Marcus Rashford (Manchester United), Jadon Sancho (Borussia Dortmund), Raheem Sterling (Manchester City), Callum Wilson (Bournemouth)