Southgate has included Manchester United left-back Luke Shaw, despite him not playing since February, but with no Ben Chilwell in the squad, if Shaw isn't fit enough to make the final cut, England will go into the tournament without a specialist left-back.
"I just feel other players in other areas of the pitch have had better seasons," said Southgate when asked about Rashford's absence.
Everton's exciting young defender Jarrad Branthwaite and Liverpool defender Jarell Quansah are both in the squad, along with Joe Gomez, who could be an option at left-back if Shaw isn't fit.
The 20-year-old midfielder Adam Wharton is also given a chance after impressing following his move from Blackburn Rovers to Crystal Palace in January.
Meanwhile, Aston Villa's Ollie Watkins and Brentford forward Ivan Toney have been given a call-up, with Toney perhaps lucky to be preferred to Bournemouth forward Dominic Solanke, who ended the season with 19 goals in the Premier League.
"There's definitely much more attacking options than compared to other squads," commented Southgate, who was also happy his players had more "tournament experience."
"We're really happy with where the squad is at. But everything starts from zero. It doesn't matter what we've done in the past," he added.
Full squad:
Goalkeepers: Dean Henderson, Jordan Pickford, Aaron Ramsdale, James Trafford.
Defenders: Jarrad Branthwaite, Lewis Dunk, Joe Gomez, Marc Guehi, Ezri Konsa, Harry Maguire, Jarell Quansah, Luke Shaw, John Stones, Kieran Trippier, Kyle Walker.
Midfielders: Trent Alexander-Arnold, Conor Gallagher, Curtis Jones, Kobbie Mainoo, Declan Rice, Adam Wharton.
Forwards: Jude Bellingham, Jarrod Bowen, Eberechi Eze, Phil Foden, Jack Grealish, Anthony Gordon, Harry Kane, James Maddison, Cole Palmer, Bukayo Saka, Ivan Toney, Ollie Watkins.