Why are their hands around their chest?
This pose indicates that they represent the dead and served to identify them with the god Osiris, the king of the afterlife. The is referred to by Egyptologists as mummiform--mummy shaped--or, especially in the case of a king, Osiride--Osiris-like.
Shabties like these would be placed in the tomb. They're essentially servants to the deceased, who would perform tasks like farming for them in the afterlife.
Tell me more.
These figurines were inscribed to the person they were buried with and were thought to come to life in order to help with chores, especially agricultural tasks.
They came in a wide range of qualities, from customized and individualized (the most expensive) to mold made and mass produced.
Do you know why their hands are crossed like this?
The hand positions of shabties mimic those of ancient Egyptian mummies and mummiform coffins. These small statuettes were meant to follow their owner into the afterlife and act as servants. You'll notice that they even carry hoes and other agricultural tools to perform their duties like the way a king by be shown carrying a crook and flail, symbols of his office.