He doesnt want to.
Xavi was not some top defensive coach either. Not by a long long way.
Took some absolute hammerings in Europe and from Real Madrid.
It was pointed out so many times here, by the same people, me included, that our goals conceded rate at that time was not sustainable and would not hold against better opposition.
We benefitted from opponents missing clear chances (which happens, sure) but more importantly, when teams started to hack it, we really struggled. Towards the end it started to become evident that Barcelona were not pushing for goals and were vulnerable if challenged for possession.
The game against United is a good example. We took an early lead, tried to hold it conservatively and recycle possession repeatedly, and they overran us and took advantage of how cautious we were. Once teams lost that "respect" for our threat in attack we started to tumble under Xavi.
With Flick we win that game by multiple goals.