Because the mother of all players is pulling 4-5 defenders to him at all times and the rest of the team cannot defend or press to save their own lives. The team has absolutely no depth. The bench is a joke.
There's little to no complaints about the team's scoring record. It's the ease with which the team concedes that keeps biting us in the ass.
In reality, Barca needs to go through a rebuilding phase, but that would completely waste Messi's last few years and would be a shame. If the club is eager to keep him, they might as well go on a spending spree. There's no point to having Messi and going down 4-0 in Paris or 3-0 in Turin.