Or…it could be bugged/badly coded.
Even in games with much much much more resources, staff, and money, things like that happen. I mean, in GW2, they had bugged drops for years and the Devs kept saying “Nope, all good” until finally someone stumbled upon the actual problem.
I myself have gotten Outstanding on Dante1/Fountain 6-10 times each, and take them every time they come up. Not once have I seen Dante2/Subterranean. I admit that luck can be bad, but I cycle 15m missions for 3-10 hours day, and have since the system launched. I think that if the “chance” is so low I have never even seen each mission, that it is either bugged…or if not, severely needs an occurrence boost.
Edit: For perspective, I have completed 2,053 Agent Missions, and never seen Dante 2+ pop, nor Subterranean, and 90%+ of those missions are Tier 1 or 3, trying to get it to pop