Maybe it would be harder these days but credit card tips to a delivery driver weren't really a thing, they dealt mostly in cash.
Arguments using past, obsolete circumstances do not apply to today's facts. Uber, Door Dash, Pizza delivery and almost every business that serves food (Starbucks, for example), solicits tips from customers -- even through the driver thru! Few transactions are all cash.
When I go to a nice restaurant like Stuart Anderson's, I always pay with a card. However, when they return with the receipt, 9 times out of 10 I will leave the tip separately in cash. This is sometimes because someone at my table offers to pay the tip for me (or I for them), but also because we know the tip system varies by store, and it's easier for the server we intended to tip to keep that tip -- or at least most of it.
If they decide to not include that cash tip in their IRS return, that's between them, the IRS, and their conscience.