Because I don't think there's data out there to that effect. As for the stringer playing it safe, in my opinion they're probably more concerned about you exceeding the recommend tension of the racquet, which is much more expensive than a set of gut. Still, I don't think 70lbs is unreasonable, but the fact is stringing at tensions that high using a 17g string increases the likelihood of a premature break during the process or shortly thereafter. But I'm sure you know that.
As for driving back and forth to the stringer, that's an easy fix: get a stringer. You impress me as a young, avid player, so take the plunge, man. I promise you'll regret that you didn't do it sooner!
Need help finding lost dog: walks with a limp, recently castrated, can only see out of one eye - answers to the name, "Lucky".