Demons seek hosts as a bridge to the next plane. Obviously, if they can manifest without one then there is no point to this, save the higher-functioning demons who might want to exist incognito.
But, even in that case, they can magically create a form for themselves without requiring a host.
Exactly. And sure, some of them will go it without hosts. But the sheer number of rifts and the breach itself along with all of the chaos associated with such events should be the circumstance of which to expect a horde of abominations. You have a horde of stressed out rebellious mages of which to see the world through and gain power from, or dead mages to puppeteer. Like Arcane Horrors that the Inquisitor encounters, while technically not abominations are close enough.
The writers obviously chose not to go that route as they may have thought it too obvious but the option and expectation of a 'horde of abominations' is there.