Gatlin: ran as oldie the same times as during his best years, which were doped. A clear career doper who didn't get caught anymore after his ban.
Rupp: likely doping, elevated testo, forbidden method, and that's just what leaked. No action from USADA.
Armstrong: again, we knew of his doping since 1999. USADA didn't act before 2011, 11 years after its start, after the feds literally made their case. If you see that as an example how USADA works, i.e. only when they really have to, then it's obvious why almost no runner gets banned.
Why I disagree with the kissing and beef excuses? Well for one, USADA itself pretended to disagree with the kissing excuse, but couldn't get bothered to build a case.
As for the beef, that was accepted by USADA, without any evidence other than the athlete claiming to have receipts for beef. Here USADA chose to trust without even analyzing the beef. Sets a terrible precedent.
@testing: that does not mean an athlete is clean. That only means he/she cannot dope above certain threshold, what people here misleadingly call gray zone or microdosing.
Let me cite from a recent article in Sports Med. (1st issue in 2018):
"One [doper] in two athletes is a big number, and suggests that doping is truly endemic in elite sport."
You guys need to stop pretending that doping is rare.