The answer to the first question will start to become obvious as I begin to answer the second question. There are many things that can go wrong with a piece of pottery during the making process. And every time a piece is lost, the potter incurs a financial loss because of it. Sometimes the piece never makes it off of the wheel because it develops an anomaly (warps, collapses, tears apart, becomes uneven, reveals air bubbles, etc...). When this happens, time is lost, and if the potter thinks of his labor in terms of an hourly wage, then he just lost money and must make it up in the next pieces he makes.
Next comes the drying phase of the process - during this period of time, the piece can dry unevenly and warp or crack. This is not uncommon and when it happens, the time spent making the piece is lost - adding to the cost of the surviving pieces. Understanding the strengths and limitations of the particular clay body can minimize much of the losses in these early stages - but it cannot eliminate it entirely.
The next stage of the process is the first firing of the pieces (known as the "bisque" firing). This first firing hardens the clay to a soft and porous "terracotta" state. At this stage, pieces can warp and crack depending on the stresses existing in the piece from the making process, and stresses placed on the piece from the heat being applied to it. A longer firing time (more expensive) with a slower heat rise (greater cost in terms of time) is less stressful on the pieces and results in less loss at this stage - but occasional breakages still occur.
In the picture on the left, two pieces were welded to the kiln shelf by a glaze combination that increased the melt and caused it to run off onto the kiln shelf. The pieces had to be broken off the shelf and the sharp fragments and glaze droplets ground off using a high speed grinding wheel. This is just one example of the many things that can go wrong. The cost and loss of these two pieces (in terms of both time, materials and overhead) has to be factored into the price of the work fired successfully and spread across several hundred pieces.
So, these are some of the things that factor into the cost of handmade pottery, and we haven't even begun to discuss all of the overhead costs of a dedicated studio building and utilities, equipment costs and maintenance, marketing and training and..... then there's taxes - don't get me started!