Maybe it's a matter of "well, if a few players know Rocketry, they won't trade it to me," thinks the AI, seeing as how those other AI would simply reply with "Sorry, we'd rather win the game, thank you very much!" when the first AI asks for Rocketry in a trade deal.
However, if a couple of other players are silly/smart enough to research Fission, then one of those who knows the tech will start to trade it around... and those who already have Rocketry in their back pockets won't be completely left out of the Nuclear Arms race.
Of course, when ALL of the AIs behave in this way, then the human player can easily get the Fission monopoly and just refuse to trade it around, muahahahaha!
Then there's the whole "I can't wait to upgrade my Infantry into SAM Infantry" line of thinking from most of the AIs, which can help against a human player that beelines Flight. Of course, in the more general case, it is a silly move against a human player that is capturing Cities with mostly land-based units (and who is possibly bombarding with naval-based units instead of air-based units).
Thankfully, the game doesn't have too many bad upgrade paths, such as if we were to introduce the idea of Infantry upgrading into Anti-Tanks. On that note, though, Grenadiers upgrading into Anti-Tanks would probably actually be a reasonable upgrade path, seeing as how the low base strength of an Anti-Tank unit is still higher than that of a Grenadier, and the Grenadier's bonus against Riflemen is of no use if the game is at a point where all Riflemen have been ugpraded and are no longer buildable.