Why is ministerial turnover so high?

The Institute for Government looks at the high rate of ministerial turnover in the UK which is now as high as for football managers. It cannot be conducive to good government: Keeping ministers in post

Martin Lewis was making the point at Radio 5 at lunchtime that he often found in his discussions with ministers on financial issues affecting consumers that the minister had not mastered their brief and he had to give them a 101 course on what the problem was.

I remember Richard Rose tackling this topic decades ago and pointing out that ministerial turnover was actually lower in countries like Italy that have frequent changes of government.