Thanks to being from the end of time and then further evolving through its beginning, Mindflayers hold incredibly advanced biology that requires specific nutrients to function and keep running. This diet of hormones, enzymes, and psychic energy is conveniently all found in the brains of intelligent creatures. Illithids will also harvest and store these brains for scientific endeavors such as the creation of Intellect Devourers. The issue is that a Mindflayer's diet requires increasingly complex brains. In addition, their paranoia, thanks to the whole Gith Rebellion phase of their empire, leaves them constantly battling between hiding from extinction and potentially revealing their hunting territory by venturing into it.