Derivative Linux distros need to keep up with the distribution they’re based on, i.e. DebianOS > PureOS, or else they run the risk of missing important security updates, package fixes, or system changes.
There are very specific reasons the particular components for the Librem 5 were chosen by Purism. This community FAQ provides a lot of the background info, especially this section.
The Linux kernel is “tweaked” out of necessity to run the hardware and the applications.
6.1. What are the Aspen, Birch, Chestnut, Dogwood, Evergreen and Fir batches?