Probably because touring is about enjoying a low intensity ride. For instance, you may want to read this blurb about RAAM & Tour de France, both long distance cycling events, but with obviously different ethos. The Tour includes 2 rest days inserted in a three week high intensity high speed event. The RAAM is stageless (i.e. no rest days) with fastest riders completing the 3000 miles event in a little over a week (!!). By comparison, tourers plan for 60 days or so to ride across America.

Resting heart rate is usually a good indicator of exhaustion. I typically tour 4-6 weeks at a time, 100kms/day. My RHR remains stable.