Borthwick picked 19 forwards and 14 backs, with more than 1,400 caps, for the tournament in France next month, but left out centre Henry Slade and loose forward Alex Dombrandt in shock omissions.
"There have been a number of very difficult selection decisions to make, given the quality of our wider training squad. Some very good players have missed out on selection such has been the competition for places over the last eight weeks and more of preparation," Borthwick said.
"Rugby World Cups involve unique challenges in terms of the demands on players, the need for squad flexibility and the limited number of squad places available.
"France 2023 is no different, but I am confident we have selected a squad of 33 that is well able to meet those challenges and leave us best placed to be at our most competitive in this exciting tournament."
On the reason for announcing his squad in early August, Borthwick added: "We believe that announcing the squad a little earlier than otherwise might have been the case, brings a clarity and certainty to the group, allowing the players as much time as possible to settle and develop as a squad."
Several players who had a chance to impress in last Saturday's test in Wales did not make the step up, although Theo Dan, who won his first cap in the 20-9 defeat in Cardiff was selected.
"As Saturday's game in Cardiff proved, we have a series of demanding test matches through August as we continue that preparation towards our opening Rugby World Cup pool game against an in-form Argentina side," added the coach.
England take on Wales again this Saturday in their second warm-up international but this time at Twickenham.
Borthwick's squad sees Ben Youngs, Dan Cole, and Courtney Lawes compete at their fourth World Cup. Youngs is England most capped player with 122 appearances.
England will meet Argentina in their opening Pool D fixture in Marseille on September 9. Japan, Samoa and Chile are the other teams in the group.
