How to DRY when building back and front-ends?

one reason they might be doing it that way - is to have separate environments. so yes its starting out the same,
but the idea is that there will be - for example - different jquery scripts for backend admin tools - versus what is needed for the public front end. using full js files for admin development versus minified js files for the production front end.

you can also extend that where - basically the admin can be the 'sandbox' for beta functionality for the front end of your website. for example going from bootstrap 3 to bootstrap 4. it would make sense to apply those changes to your admin first - make your mistakes in private - and then upgrade the public site.