The lorebook section on voidsent explains how the "needing a body" thing works. There are basically two ways that a voidsent can leave the void: coming through a voidgate (whether naturally formed or deliberately created to summon a voidsent) in its own body, or just its soul can pass through and possess "a 'vessel' provided by the summoner". The "soul method" requires a smaller hole than an entire body, so larger voidsent are easier to summon this way.

Natural aetherial tears are rarely large enough to let major voidsent through, although "in the years leading up to and following the Calamity, the barrier between the two planes has grown weak, resulting in larger and more frequent tears".