There isn't anything special about Pizza delivery that makes it cheaper to do than UberEats or Instacart. The only difference is the margins. Pizza companies can make a $15 pizza for ~$3, and that means they don't lose money even if they spend up to $12 delivering it.

Instacart, on the other hand, likely doesn't have any margins to play with at all, considering the fact that they are paying people to shop and check out at grocery stores. And the stores already have low margins (~1%) which means they likely don't have much in the way of sweetheart deals with the stores. They are likely in the red before the goods even leave the store.

Some restaurant foods might come close to breaking even, but few restaurants approach the margins of pizza-making at high volumes.

But don't worry. There isn't any indication that VC's have figured out math yet, so we aren't in any danger of these services going away.