They'll eventually bush out. You can pinch out the top to make them bush faster.

Thanks. I'll try to be patient. Also, we have fairly alkaline dry soil here. Would it work to mulch with pine needles?

If you have any oak trees around, shredded oak leaves would provide a bit more acid to the soil as they break down.

This sounds familiar.
Last year I grew some HH from seeds, but they seemed to "disappear", at least went unnoticed in the back of the bed.
This spring I was cleaning out the bed and was pulling all these "twigs" at the back of the bed when I realized the placement of the twigs coincided with where I had planted the hibiscus.
Stopped pulling, and now I have quite a number of new HH plants!
Moral: be patient!

They do tend to look very dead come springtime, but they only die back to about ground level or sometimes a little deeper, and were you to dig around the old stems , as you got near the roots, you'd see some green still at the very bottom of the previous years growth, and thats where the new eyes form to grow into each new years growth. They for the most part never do become multibranched plants like some other perennials, but the clump will increase from year to year to give you lots of flowering stems, so you need not worry about how much each individual shoot branches to get lots of flowers. There's always one shoot the first year from seed, but the following year there are ususally more than one new shoot that come up from last years shoot. Sometimes not evidently from what you've said, but that should change next year. After a number of years there may be so many crowded shoots, that they struggle to compete for space and you may consider dividing them into smaller clumps in the early spring. It's a nice way to increase the numbers , without having to start them from seeds all the time.