Why is butter incorporated into the dough last when making Brioche?
The reason is that butter can inhibit gluten formation.  It 'coats' the proteins that would form gluten.  You knead the dough first to get gluten, and then add the butter afterward around the already formed gluten.

You can add it earlier, you just end up with less gluten and a more tender dough.

Creaming isn't usually done with bread, as its purpose is air bubble formation.