All the sub par 70 courses in GB&I mentioned so far were built in the days when "bogey" was the standard way of assessing how many stokes it should take a good golfer to complete a hole. Holes much over 400 yards were generally classed as "bogey 5" holes.
"Par" only came into common usage between the wars. Many older courses with a perfectly respectable bogey of 73 or 74 found themselves with a par of 68 or 69 under the new system. Courses built subsequently have generally had a par of over 70 as this became thought of as the baseline for a serious golf course.
Many courses have extended a couple of longer holes to reach par 5 status and so tip the course over the "magic" 70 figure. Many others haven't whether through choice, obstinance, or lack of space.
The change in meaning for the term "bogey" to one over par only became widespread in GB&I after WW2. Some of the more traditional clubs were still using bogey instead of par on their scorecards right up to the end of the 20th century. I suspect that one or two might still to this day!