Mobs don't need a visible spawner to spawn, just certain conditions need to be met like mob density per loaded chunk, time of day, nearby structures like towers or visible spawners.
The Draugr spawners you can see which are called body piles for instance won't spawn more than 2 each and only usually have 2 mobs each when you are close enough to them.

So yea theres a never ending tide of mobs in every biome, they can spawn behind you as you advance too