You’d think that, but it’s not actually true.
Consider what happens if a person gets banned for botting, for example. Their accounts are locked, including any premium currencies they paid for out of pocket. But cheating in a video game isn’t actually a crime anywhere (at least in Western countries, as far as I know), so CCP actually has an obligation to provide the goods/utility that was paid for, or refund the money. But they won’t. That PLEX will sit inside those banned accounts, and possibly be used as part of the stock that they use to balance the PLEX market (at least, that’s what they say).
So PLEX isn’t truly more safe than other assets; it’s just that if anything happens to it, it won’t be caused by other players, but by CCP themselves. Consider a hypothetical situation in which the player base is dropping, and they can’t stem the tide the usual way. I wouldn’t put it past them to do something like attaching an expiration date to PLEX in order to entice players who are away from the game to come back to use up their stocks, lest they lose them forever.