You failed to mentioned, that Liverpool also has most clean sheets in the league. So robertson must be doing something right to contribute to that. In a system where both fullbacks bombard forward on every chance they get, defending is lot harder than defending in static systems such as used by LVG and Jose.
One misconception here is , that shaw is better in defensively but robertson is better offensively. So according to you, if you put shaw in liverpool's team, he will be able to contribute offensively but what about his defending? Shaw could barely finish a match in jose's system, system in which Man utd team was one of lowest teams to clock kms in game on average while liverpool under klopp has been in top 3.