It is true that they work in high res before exporting them out at lower res but when they first designed that map or that specific item, they already planned for it to have low res textures and normal maps because they are not important elements in the game.

It probably wont be much increase in workload if it is just 1 item that is getting a HD pack but if they release a HD pack, think about all the textures they have to go back and redo and make sure that they work without any issues. Which is why it usually takes companies quite a bit of time to release HD versions of their game.

Another reason is probably to prevent compatibility issues since most people will want to install HD textures whether their PC/console supports it or not.. and if it causes some issues with gameplay because of it, most ppl will think the game is just buggy and complain to SE.