Because a guy who went to a Uni for five yrs can always perform just the same as a coach who has handled coaching both men and women's teams for 30 years just the same. That is why it happens so often. Like all the former Arkansas guys who went into coaching so successfully, or so many of the 300 people who went through the Wisconsin program the last 30 years. Many of them have gone on to coach just as effectively as McClimon, and Martin Smith, and Jerry Schumacher, and Mick Byrne. Think of all the excellent coaches that used to run under Harry Groves? Because it is easy, and YOU see no reason why it won't keep rolling right along.
I cannot think of anybody who coached an AR-level, medal-winning athlete who hadn't already coached at the HS-level and/or the college level FIRST, and whose main claim to fame was "I was a college runner".
There are several former college good/great runners who are or were coaches (Robert Gary, Salazar, Schumacher, Tyson, Powell, Jim Stintzi, Ken Popejoy, Jim Spivey, ... there are lots). But not one of them began their coaching stint with the all-time best American in their event, who is also their wife.
You are just naive to think this is normal or is not fraught with peril. How many men have their wives coach them, just because their wife was once on a team that qualified for NCAA XC Championships?