I think Instacart's other justification for this is that multiple people are now involved in procuring your groceries. One person shops and another delivers. If

I think Instacart's other justification for this is that multiple people are now involved in procuring your groceries. One person shops and another delivers. If you tip in cash, the shopper gets nothing. Can't speak to if this is their real motive, but does seem to be in line with other criticisms of tipping (e.g. waitresses making much more than kitchen staff).