This is just not realistic.

About sets: building them is hardly the solution when you have so many... It's even more expensive than digital set extensions. GOT limits massively the number of sets, WOT past a certain point has many more locations to cover through one "season". Unlike GOT, it's far more urban.

As for crowds...

The salaries of the extras is but a small fraction of why big scale scenes cost so much.

You need big sets to use a lot of people in, and extra lights (you need to see the negotiation going between a director of photography and a production manager for each extra light to realize just how expensive that is). You need many extra set technicians to work on that scale. You need costumes for the extras, and props. You need additional camera teams on set to film big crowds, and all kind of expensive gear and coordinators to coordinate their efforts and make sure you get everything you need. You need extra production/director assistants to control and guide those crowds extras, even more when you work with non-professional extras (it makes production companies fairly insecure to work with non-pros. They do it when they need several hundreds and more, but they don't like it one bit). You can't just use anyone for battle scenes, you need trained extras - remember LOTR's huge scale bootcamps, and they used mostly people with martial arts training to speed things up. There are work laws, and union regulations. You need to feed those people, pay for security services, pay for mandatory emergency medical care on set (the more people, the more medics you need on stand-by... you're American, you must have an idea of expensive that gets...). You also need extra insurances for liabilities when you deal with crowds, especially for battle mock-ups, you need many extra stunt coordinators to keep everything safe. If you need to involve horses or other animals, it gets far worse still.

That's why more and more movies rely on crowd simulation softwares for big scale scenes, but those VFX are still massively expensive to use, that's why we still mostly see them only in big budget movies. One day maybe... 10 years ago having a CG wolf on TV was completely out of question, now a big budget mini-series can afford some "hero shots" of a wolf (and it was in part to cut production costs...).

The casting problems are not merely because with such a huge cast you can't afford big names. It's largely a logistic problem. The bigger a cast, the larger the logistics problems to organize filming. The main and secondary cast are under contract, they're not the problemn (though for secondary players, you need to pay them well not to have any). In WOT the problem would be the sheer number of "guest star" type parts. They keep returning, often whole "seasons" apart. Elaida, Sheriam, Verin, Domon, Siuan until the mid-series, Thom until the 3rd book, Lan after TFOH, Moiraine is gone for several seasons. There are dozens of those, few of those comparable to Glorfindel. Those actors you can't have/keep under contract once they stop being regulars and in some cases until they become regulars once more. You must work around their various commitments and schedules. That's how it's done. E.g.: before they could bring back Rousseau in Lost, write her in further episodes, they needed to make sure Mira Furlan was available again and when, so they can determine in which episodes they could write her in. Before the writers know if they can bring back dead characters 2 seasons later in Fringe, they need to secure the participation of the actors again. Sometimes it works, sometimes it just can't work and they must go in other directions. The more you rely on occasionally recurring guest stars, the more compromises you have to make (eg: you can't have Furlan for episodes 5 and 7, you need to change the story so you will need her in episodes 6 and 8 instead. That would make any network and production company extremly nervous the tertiary cast is recurring on such a large scale and for most with huge gaps between each appearance. Re-cast is possible, but very impractical for side players - the recurring actors is after all how you can avoid having to reintroduce those characters in a more involved way each time. If you're adapting a story, that's a nightmare. I can't think of any TV show that even attempted to work on anywhere close to that scale, casting wise. You cut the WOT cast by half among the recurring tertiary players and it would still be bigger than any show.