Most likely because it's a reprint of an old adventure where DCs would scale by level. Some of the dungeons in it probably have as much if not more treasure than at least one full (likely more than 1) 5e HC adventure too. 5E has design elements that make certain styles of play somewhere between difficult to problematic & the inability for wotc to convert those old modules over without losing a lot in the adaptation if they changed them to fit 5e norms highlights that.IMO, its mostly terrible adventure design...
Why are the DC's for doors so high? Assuming that a character has +2 Prof, and +2 or 3 for their Wis, they have to roll a 15 or so to see the door? Is it intentionally so well hidden as to need that level of Perception? If not, bad design. If so, why?
And are there other ways to access the areas hidden behind the doors that are less onerous? Why not?
Is accessing those doors critical to the adventure's success? If so. Bad design.
And frankly, half of those rolls are stupid. I have to roll a DC 15 to find a jade figurine in a firepit? Or to notice missing mortar? So I have a 50% chance of missing those? Bad design. They could both be auto successes if someone pokes through the firepit, or look around where the mortar is.
With adventures like this, no wonder people in the other Perception thread said "I roll Perception". It seems that at least this adventure is modeled on Perception checks everywhere, and a lot of times where they aren't needed.
YMMV
DC15 is pretty low for a check with proficient skills not tied to your dump stat though.