For anything that's even somewhat in the consumer space rather than pure workstation/professional, the main reason is that dongles can be used with a laptop but add-in cards can't. When ordinary consumer PCs (or even office PCs) are in the picture, laptops are a huge chunk of the target audience.

The market segments that can afford to ignore laptops and only target permanently-installed desktops are mostly those niches where the desktop is installed alongside some other piece of equipment that is much more expensive.