Some quotes from various writers that should give some perspective:
Tilly wrote in the American Cricketer:
"...Merion is at present in a very early stage; consequently we must regard it as the foundation from which there will gradually rise the structure of the builder's plans. To attempt an analysis of some of the holes today would be manifestly unfair, fo rthey are not nearly so advanced as others, and yet some day the very holes which now are rather uninteresting and featurelss may be among the best of all."
..."Even after the course is thoroughly trapped it will be difficult to figure the part figures at more than 70, and yet few players will approach this score."
..."As I have said already, comparatively few traps have been placed. The Committee wisely desieres this to be the work of time. The Mid-surrey scheme of grass hollows and mounts has been introduced in some places, although not to a considerable degree. Every hazard is more or less experimental, and when the real digging is started, the pits and mounds will be sufficiently terrifying, I am told."
Alec Findlay wrote the following for a local paper:
"There are a few nice water hazards, and also a few sand ones, but the placing of the mental hazards, etc., will be left until spring. One can by that time find places wherein shots will lie, and place hazards accordingly."
And this from Far and Sure upon opening;
"It is too early to attempt an analytical criticism of the various holes for many of them are but rough drafts of the problems, conceived by the construction committee, headed by Mr. Hugh I. Wilson. Mr. Wilson visited many prominent British courses last summer, searching for ideas, many of which have been used."
Now concerning Cobb's Creek, which was being built starting in 1914 or early 1915, I believe:
This from Verdant Greene of the Philly Inquirer, shortly before Cobb's Creek opened:
"...but with good weather, May 1 will see little more to be done except in the way of putting in traps and bunkers, a job likely to require two or three seasons. As there are an abundance of natural hazards such as trees, water and boulders no one need worry over the lack of artificial embellishment thus far."
..."However, it is probably that some of the trees and stones will be eliminated to compensate for the pits and bunkers added. Bothersome as some of them will prove, it was best that they should remain indefinitely, inasmuch as several months experience may be necessary to demonstrate beyond dispute just what hazards should be added and where."
Another article a month before CC opened said:
"Most of the trapping and pitting will go over til next year, as those in charge do not think it advisable to make the course too stiff a proposition in its early stages. The rolling country which makes up the course and the fact that the creek guards quite a number of greens are sufficient at present to make the course difficult. As hundreds of golfers who are just learning the game, not to mention other hundreds who will learn the game at Cobb's Creek will make up the bulk of the players, a too stiff course would hardly be suitable."
And one more:
"Very little bunkering has been done yet, and the course will not be made harder for a year or so, for those who laid it out realize that it will be played over by a host of persons who have never played golf before, and no effort will be made to make it too difficult for them until they have reached the point where their golf will admit of stiffer bunkering."
The same thing was true at Seaview.
I believe some thought the idea was to wait to decide where hazards were best placed after seeing a course in action. And with Merion and CC, both after opening the amount of play they received was very high (Cobb's almost completely with beginners, but if you believe the first article above, Merion had plenty of newbies as well) and the natural hazards present made them plenty hard for the majority of the players. And maybe this played a role in Merion appearing to be slow to add more bunkers.