I think an economist would tell you it is human irrationality. However, I think there are a few factors - a big one is that a change is as good as a rest. Doing 40 hours development and 10 hours running errands is a nicer balance than 50 hours development, and probably with similar productivity. There's an overhead to hiring help too - setup and management. You're also becoming more removed from everyday life. That might be a benefit or a drawback.
That said, I recently hired a servant to take care of all those things that I don't like to do (I live in a low-wage country). Shopping, Cleaning, Laundry etc. About $400 a month, although it could be a lot less. Wouldn't go back.
Grocery bagging specifically is an interesting point culturally. Bagging helpers do exist in the UK, but 99% of the time you will bag your own groceries as the cashier scans them. It gives you something to do while waiting, and enables you to pre-organise the bags any way you like.
There is something of a time loss if you can't keep up with the cashier, but that's not often much of a problem. The few times I or my husband have started bagging stuff in grocery stores here in California, though, we've gotten strange looks - and often, at Trader Joe's, where the cashier often bags after scanning everything - words of thanks.
Now, interestingly, I really shouldn't be there. The cost of shopping for groceries on a whole is _very_ expensive, time wise. I would rather sit at home (or do something else productive) than walk around the grocery store and wait in lines for 45-75 minutes.
However, I still don't use grocery services, and I know very few people per capita willing to let others shop for their groceries, even though many of them bring home over $75 an hour. Why is this?