Pure speculation here: I think the difference may be in the specific timing sequence of entering the jhanas vs e.g. drugs. With drugs, there's a known correlation between the immediacy of getting high after taking them vs addictivity (injecting is more addictive than snorting, which is more addictive than ingesting, even for the same substance), which reliably forms a craving-action-reward feedback loop. With jhanas, entering them requires a certain relaxed state, which (I think) necessarily involves letting go of craving _before_ you enter the jhana, and so there's no feedback loop.
Pure speculation here: I think the difference may be in the specific timing sequence of entering the jhanas vs e.g. drugs. With drugs, there's a known correlation between the immediacy of getting high after taking them vs addictivity (injecting is more addictive than snorting, which is more addictive than ingesting, even for the same substance), which reliably forms a craving-action-reward feedback loop. With jhanas, entering them requires a certain relaxed state, which (I think) necessarily involves letting go of craving _before_ you enter the jhana, and so there's no feedback loop.