> Why don't they just ship it with W10 Pro already installed?
Are you sure they don't? In the past, many Windows edition upgrades (e.g., Home to Pro) were just a matter of enabling features that were already installed.
This is me being pedantic; when you upgrade from Home to Pro it does actually install several missing features. You'll see the this[0], takes about 30 minutes on an SSD.
[0] https://www.technorms.com/assets/20151012_133129.jpg