My brother was a DoorDash driver and a lot of his friends were and they all said that they regularly skipped orders with $0 tips because it meant they made nothing on them. DoorDash, at one point, even tried to implement something to address this by changing the way the income for the order was displayed but it's still quite prevalent.
So, yeah, tipping cash is a great idea except these companies have built their whole model in a way that makes that look like you're going to tip $0. So unless someone doesn't care and picks it up, in my area at least, it sounds like you're just not going to get stuff delivered.