its typical to have medium swamps with 0 - 2 crypts and yet elsewhere a smaller swamp may have 10 or so. Every map you generate is like this, give the map generator a try and even if you don't put YOUR map into it, you will see how bad the distribution of these (and ice caves later) can be with none at all in such large areas.
I personally use the map. I know it can be fun to run blind and explore, but I don't have the 20+ hours to find a useable area these days.

I wish they would look into this just a little, to make sure a good sized area has a few, or alternately make the random outdoor piles a little more frequent & higher yield. Iron is arguably the most critical asset in the game, used for structural integrity and over half the armor and weapons and many of the crafting stations use a good bit. Not finding it after a while is game breaking.