Celtic are victims of their own success in Scotland.
They basically don't have to do any defending at all, and if a team shocks them by taking the lead they just switch on and score three or four in return.
In Europe they can't defend because they never have to do any defending domestically.
This is shown by other Scottish clubs who are proven to be many, many, many times worse than Celtic domestically but they don't get battered in Europe as often
Keeping games tight and boring like you need to do away from home in Europe is something that they just can't do, it's not instilled or needed to be practiced by the team