purplelass I think the problem with that is storage. For example, a popular brand of cereal like Weetabix. If a supermarket has 2 cases of 12 on the shelf, that's only 24 customers served.
If that one supermarket sells 3 times that in a day that's 72 boxes. To cope with a truck being one week late they would need to store 504 boxes of Weetabix.
If that one supermarket sells 20 brands of cereal that's potentially 10080 boxes of cereal.
Then chuck in 20 types of pasta, 10 types of rice, 5 types of pulses and lentils, couscous, tinned fruit, tinned veg, tinned meat, coffee, tea, sugar, flour, dried fruit, and so on and on and on.
There's only so much storage space. There will be contingencies but even the big 4 have their limits.