Pre-carved stocks are harder to work with than starting from a blank. Also pre-carved stocks are limited to what is commercially for sale. With a blank you can build anything that you want. Pre-carved stock will only work with a specific barrel, lock, buttplate, etc. Pre-carved stock limits your choice of wood to what the supplier has on hand. This one is just an average piece of maple. Not a big deal. But when you are looking at highly figured Turkish walnut that costs a few thousand dollars, there is no pre-carved option. Pre-carved stocks have a fixed length of pull. They also usually have too much distance between the barrel and ramrod.
Pre-carved stocks take away almost all of your choices in what you can build and add a lot of extra problems. Nothing is squared off on a pre-carved stock. That makes everything far more difficult. When you build from a blank you have no limits to what you can build. Any errors will be the ones that you make yourself. There is no advantage to working with a pre-carved stock and many disadvantages. Many people seem to think that it is an easy way to learn. It is not. They look almost finished so it seems like less work. That is all in your head. They are 90% inlet. You only have 90% left to go. But the real problem is when they are 105% inlet. That happens a lot.