While the elevation gain and distance on the trail may not seem a challenge, the high altitude environment poses different challenges. The thin air slows you down so the climb takes longer and can cause altitude sickness. Sunburn comes quickly. The weather changes even quicker. In Colorado we put an emphasis on climbing speed that newcomers don't understand, it is the weather.
Lightning is very much a danger on the fourteeners, increasing in probability latter in the day.
The general rule is to be down off of the summit by noon. So while it is 90 degrees in Denver, it is about 45 degrees on the fourteeners, prime temperature for hypothermia, especially if wearing the typical tourist uniform, cotton. See what to bring on a hike . Most summits cannot be reached by a trail. And then there are summits that will never have a trail built on them, on these the climber is exposed to serious injury or death if a fall should occur.