I'm unclear why I would tip my Instacart delivery person. I order A, B, and C; my expectation is that A, B, and C show up, because that is what I paid for. How can a particular delivery person do that "better" enough to deserve a tip?
To your point, I have noticed that Instacart users fall into two groups, those that care about specific replacements when an item is out of stock, and those that just want the shopper to use their best judgment. If you care about replacements, then you want your shopper to be responsive and well-mannered in working with you to resolve the shortfall.
I'm not saying the lack of tips doesn't hurt drivers, but I agree that I have no idea how I could determine when a tip is justified.