The main reason is that desktop PCs typically use off-the-shelf components that are much more plentiful and cheaper. It does not require a special case, board and cooling solutions for that particular model.

Desktop PC components are in much higher demand, and many more are manufactured. This means that the final price is usually cheaper

The equivalent laptop components are smaller or require different technologies, and therefore more expensive to produce.

Design and assembly is also much more difficult on a laptop, and therefore more expensive.

In addition, there are specific components such as batteries that are not used in desktop computers.