Because Southgate has always believed Phil couldn't play in the middle, based on Pep not trusting them there until this season. He doesn't seem to care that Klopp doesn't play TAA in midfield or that Bellingham has played a box to box role and deeper role quite regularly from Birmingham-Dortmund and even at Real.
We don't need to shift Saka. The biggest issue last night was that we dropped off and then played it long whenever we had the ball. Bellingham would be much better playing alongside Rice, picking it up from deep where he's unlikely to lose it because of his strength on the ball. He can still time his runs into the box, with Rice covering the space, but he's also there to ensure we can play out from the back. Foden's also intelligent enough to drop in and rotate, and it enables him to have an option left (Gordon/Eze) and right (Saka) as well as Kane to play off.