What is so unique about ushabti that makes them valued and popular?

Just to be clear what we're talking about, a shabti figure was a funerary figurine used in Ancient Egypt. Strictly speaking, we should only use the term "shabti" for those figurines which were inscribed with Chapter Six of the Book of the Dead (or, more accurately the "chapters for coming forth by day"). Figures lacking that inscription should probably just be called "funerary figurines".

The question of "value" is complicated and multifaceted. For example, the value to the tomb-owner is not going to be the same as the value to a modern collector. I will try to address the different aspects below.

First though, We need to think about the question of uniqueness. Were ushabtiu figures really unique?

Actually, the answer to that is that it varied according to the period and the materials being used. Ushabtiu (figures inscribed with Chapter Six of the Book of the Dead) were used for about a thousand years - from about the end of the First Intermediate Period (about 2050BC) until the 21st Dynasty (about 1070BC). Funerary figurines (without the inscription) were used for even longer. It is, perhaps, not surprising that there is considerable variation in the form of, and materials used for, ushabtiu over that timescale. Ancient Egypt Online has a good, if simplified, overview of the evolution of the ushabtiu over time.

Carved figures, such as those made from wax, wood or stone, were - inevitably - unique. It is virtually impossible to create an exact duplicate. Figures cast in moulds, such as those in clay, terracotta, or bronze would - by definition - be exact copies of each other, although they might be distinguished from one another by subsequent decoration, if any was applied.

So what about the value of the ushabtiu?

For the owner of the tomb, the ushabtiu were expected to carry out the onerous duties in the afterlife that they did not want to do themselves. In earlier periods where ushabtiu were provided with tiny tools etc. with which to complete their tasks, it seems that the roles of the ushabtiu servants in the afterlife may have been specific to each shabti figure. Otherwise, the ushabtiu seem to have been intended as generic servants. Here, the "value" of the ushabtiu was that they would relieve the tomb owner of the need to work in the afterlife, and the uniqueness probably wasn't a significant factor (except, perhaps, for the early ushabtiu).

One might argue that the ushabtiu figures also had a "value" for the wider society if they replaced the requirement for the sacrificial burial of servants with the deceased (something for which we do have some limited evidence from the pre-dynastic and early-dynastic periods).

For Egyptologists, the "value" of ushabtiu figures lies in the information that can be extracted about ancient beliefs, manufacturing techniques and technologies, fashions, etc. Here, the uniqueness is obviously a factor and we might expect to learn more from ushabtiu with unique characteristics than we would from mass-produced ushabtiu.

Clearly, Ushabtiu figures also have a "value" to modern collectors of antiquities. The use of the phrase "valued and popular" in the question may suggest that this is the "value" of interest.

For modern collectors, there are only a limited number of ushabtiu figures legally available to purchase on the antiquities market (even on sites like eBay), and many of those are probably fakes. Even though ushabtiu were produced in enormous numbers in Ancient Egypt, the vast majority of these are now held in museum collections around the world.

The fact that they are anthropomorphic and that they were so ubiquitous makes them popular. They are a very human link with the past. Their antiquity, and the limited supply makes them valuable.

The more unique the figure (e.g. the early figures with tiny tools to help them perform their role), the more we can identify with the individual that it was intended to represent, and so the more popular it becomes. These ushabtiu are much more rare, and - as one might expect from the laws of supply and demand - consequently more valuable to collectors.

It is this last aspect of "value" that fuels the market in illegal antiquities and has led to the looting of many ancient burial sites in Egypt - not just for ushabtiu, but also any other antiquities that can be stolen from the burials and sold.