[danielklnstein]

As another comment explains - the vast majority of drivers are not compiled as part of a standard Linux distribution.

If you're asking why so many drivers are included in Linux's mainline repository and not maintained off-tree - the major advantage of being upstreamed is that changes to the kernel have to take your driver into account as well. e.g. anyone who changes any APIs will have to update your driver as well.

Moreover, being upstreamed certifies that your driver has gone through a non-trivial review process to be merged into the mainline and is an indication of quality/stability.