Well a brisk walk (from memory) is about 15 minutes a mile.
So people under the age of 10 and over 60 probably start having a non-negligible numbers of people who can't complete a mile in 15 mins.
And then you have an obesity + overweight rating of 57.6% in the USA according to Wikipedia (and this may not cover people who are not overweight but physically fit) and you can start seeing how maybe not everyone can do even a 15 minute mile walk.
For the record I know BMI is a bit of a weak metric for covering obesity given that anyone "swole" will have a very high BMI...
”Race Walking is a progression of steps so taken that the walker makes contact with the ground, so that no visible (to the human eye) loss of contact occurs. The advancing leg must be straightened (i.e. not bent at the knee) from the moment of first contact with the ground until the vertical upright position.”
and also states:
”All the Judges shall act in an individual capacity and their judgements shall be based on observations made by the human eye.”
Of note here is the “to the human eye” phrase. If one were to introduce electronic aids similar to those used in fencing to detect whether there always is a foot on the ground, speeds would decrease significantly.