Why is it so hard to find pesto sauce in supermarkets?

Because it does not keep long. Jarred pesto does not taste very good.

The quality of pesto in supermarkets ranges from poor to even poorer, even in Italy.

A classic pesto contains only basilico leaves, garlic, olive oil, pine nuts and parmigiano reggiano and grana padano cheeses.

The more the ingredients stray from those above the less it is a classic pesto.