Well money is one thing, A-list actors aren't cheap and before you pull up the whole "1 Billion" argument there are a whole lot of other things that money is spent on other than just actors, like props, location permits and vfx artists among other things.

Plus as others have mentioned it gives a chance for newer talent to flourish, Owain Arthur (Durin), Sophia Nomvete (Disa), and Ismael Cruz (Arondir) have all played their characters fantastically.