That's enshittification for your use of the service, and enshittification for the workers, too.
The service provides a large pool of people that ensure the service is delivered. However, I have a preference for a certain group of people to provide that service. I tip well and consistently get service from my preferred shoppers. When they don’t take it, it falls to the general pool and is picked up quickly.
I personally just care that my groceries get shopped for me. Having someone who already knows my preference is just a small perk.
I don't worry about which USPS mail carrier delivers my mail -- I know it will be consistent and good enough. I happen to know who my usual carrier is, because I work from home and she likes to say hi to cats if they are in the front window. I also know the face of the usual UPS driver and the usual FedEx driver; they aren't here 6 days a week, but often enough that I recognize them.
In none of those cases do I expect a quality change based on the driver. I expect competence, and I get it so often that the exceptions really stand out.
From the Shipt workers' perspective, they now need to worry about customers discriminating among them rather than just getting the job done.
These Shipt people, though, have to interpret your preferences and essentially act as your agent as they decide what to pick from the store shelves on your behalf. Sometimes they make decisions that you probably would have made, sometimes less so; sometimes they’re confident that you understand each other, sometimes they’re nervous and want to hassle you about each of 10 different little decision points. When you find somebody I work well with, isn’t it a positive that you get to try to keep that relationship for future transactions? Isn’t this the same dynamic underpinning virtually every in-person service, from your hair cutting human to the tradies who do work on your house to the dry cleaner?
For that matter, doesn’t it create a perverse incentive if worker doesn’t believe that trying to understand my preferences will ever pay off? That it’s a one-off game rather than an iterated series of games, and effort to excel and bring human judgment to bear is wasted because there’s no way to reward it?
Doesn’t the enshittification tend to require as a prerequisite that a platform is successful at alienating service providers from service recipients (and from each other) like that?